CRISPR/Cas9 Gene Editing Technology: Experimental Design, Editing Validation, and Functional Research Strategies

CRISPR/Cas9 is currently one of the most widely used targeted gene editing technologies. It can generate DNA breaks at specific sites through RNA-guided Cas9 nuclease activity and achieve gene knockout, targeted knock-in, point mutation construction, and functional screening by using the cell’s endogenous repair mechanisms.

Keywords: CRISPR/Cas9; gene editing; sgRNA design; Cas9 nuclease; gene knockout; HDR knock-in; off-target effects; functional validation

1 Technical Principles of CRISPR/Cas9

1.1 RNA-Guided Targeted Cleavage Mechanism

The CRISPR/Cas9 system mainly consists of Cas9 nuclease and guide RNA. sgRNA guides Cas9 to a specific genomic site through complementary pairing with the target DNA sequence. After Cas9 recognizes the PAM sequence, it cleaves the DNA double strand near the target sequence. The cell then repairs the break through non-homologous end joining (NHEJ) or homology-directed repair (HDR), thereby producing different forms of gene editing outcomes.

(1) sgRNA determines the targeting position

The targeting sequence of sgRNA determines the Cas9 cleavage site. An effective sgRNA should meet requirements such as high targeting efficiency, low off-target risk, coverage of the target transcript, and suitability for the experimental purpose. For gene knockout experiments, sgRNA design focuses more on whether the coding region can be disrupted. For knock-in or point mutation experiments, sgRNA design focuses more on whether the cleavage site is close enough to the target editing region.

(2) PAM determines the editable range

Cas9 is dependent on a PAM sequence. Commonly used SpCas9 usually recognizes the NGG PAM, so whether a suitable PAM exists near the target region directly limits editable sites. If the target site lacks an NGG PAM, SaCas9 or other Cas9 variants can be considered, but PAM compatibility, vector capacity, and off-target risk need to be reassessed.

(3) DNA repair determines editing outcomes

After Cas9 cleavage, cells can generate indels through NHEJ, or achieve precise replacement or insertion through HDR when a donor template is present. NHEJ is commonly used for gene knockout, while HDR is commonly used for tag knock-in, point mutation, and reporter gene construction. Because different cell types, cell cycle stages, and delivery methods affect repair pathway preference, the experimental design should first clarify whether the goal is to “disrupt a gene” or “precisely rewrite a sequence.”

1.2 Main Editing Types of CRISPR/Cas9

CRISPR/Cas9 is not a single knockout tool, but a gene function research platform built around targeted localization and DNA repair. Different editing types correspond to different experimental purposes, and their validation priorities also differ.

Table 1 Main CRISPR/Cas9 editing types and applicable scenarios

Editing type	Technical logic	Applicable questions	Core validation
Gene knockout	Indels are generated through NHEJ after Cas9 cleavage	Determine whether loss of a gene affects a phenotype	Target-site sequencing, protein loss, functional phenotype
Targeted knock-in	HDR is performed using a donor template after Cas9 cleavage	Insert tags, reporter genes, or selection markers	Junction PCR, sequencing, expression localization
Point mutation construction	Target mutation is introduced into the donor template	Construct disease mutation or structure-function models	Allele sequencing, clone genotyping, functional validation
CRISPRi	dCas9 does not cleave DNA and reduces transcription through a repressive domain	Study essential genes or reversible gene silencing	mRNA, protein, and phenotype detection
CRISPRa	dCas9 recruits activation domains to enhance endogenous gene expression	Activate low-expression genes or regulatory networks	mRNA upregulation, protein expression, and downstream response
CRISPR library screening	sgRNA libraries generate systematic perturbations in pooled cells	Screen drug resistance genes, pathway dependencies, and synthetic lethal relationships	sgRNA enrichment, candidate gene rescreening

2 CRISPR/Cas9 Experimental Design Workflow

2.1 From Research Question to Editing Strategy

A CRISPR/Cas9 experiment should start from the research question rather than directly entering sgRNA design. If the goal is to verify whether a gene participates in cell proliferation, apoptosis, or drug response, gene knockout is usually prioritized. If the goal is to study a mutation site, phosphorylation site, or protein tag localization, targeted knock-in or point mutation construction is more suitable. If the target gene is essential for cell survival, CRISPRi or an inducible system is often more appropriate than direct knockout.

(1) Clarify the editing target

The editing target should be specified down to the gene, transcript, exon, mutation site, or insertion position. Simply writing “knock out a gene” is not enough. Before the experiment, it should be confirmed whether the target gene has multiple transcripts, homologous genes, alternative splicing, and whether the target protein contains key functional domains.

(2) Select the editing route

Gene knockout, point mutation, tag knock-in, and transcriptional regulation require different Cas9 forms, sgRNA positions, donor templates, and screening strategies. Choosing the wrong route can reduce experimental efficiency or even lead to editing results that cannot be interpreted.

(3) Establish validation layers

A complete CRISPR/Cas9 experiment should include at least DNA-level validation, RNA- or protein-level validation, and functional-level validation. If the conclusions of an article or project depend on a specific phenotype, multiple sgRNA repeats, rescue experiments, or off-target site validation should also be added.

2.2 Standard Experimental Workflow

A CRISPR/Cas9 experiment can be summarized as five stages: design, delivery, screening, validation, and functional analysis. Each stage may affect the final conclusion, especially sgRNA design, cell delivery efficiency, and single-clone screening quality.

Table 2 CRISPR/Cas9 experimental workflow and key control points

Experimental stage	Core task	Key control points
Target design	Determine editing region and sgRNA	PAM, exon position, transcript coverage, off-target prediction
Editing system construction	Select Cas9 form, sgRNA, and donor template	Plasmid, mRNA, RNP, viral vector, or HDR donor
Cell delivery	Introduce editing components into cells	Transfection efficiency, cytotoxicity, duration of Cas9 expression
Positive screening	Enrich or isolate edited cells	Antibiotic selection, fluorescence sorting, single-clone picking
Molecular validation	Determine whether editing has occurred	PCR, Sanger sequencing, amplicon sequencing, junction PCR
Expression validation	Determine whether target gene function has changed	qPCR, Western blot, immunofluorescence, flow cytometry
Functional validation	Determine whether editing explains the phenotype	Proliferation, apoptosis, migration, drug sensitivity, reporter gene, rescue

3 sgRNA Design and Target Selection

3.1 sgRNA Design in Gene Knockout

Gene knockout experiments usually aim to generate frameshift mutations through indels, causing premature protein termination or deletion of key domains. sgRNAs are usually preferentially designed in common exons, exons near the beginning of the coding region, or regions encoding key functional domains. If the target site is located in a non-common exon, some transcripts may still express functional protein, resulting in an unclear knockout phenotype.

(1) Prioritize common exons

When the target gene has multiple transcripts, sgRNA should preferably cover coding regions shared by the major transcripts. If the research object is a specific splice isoform, it should be clearly stated that the target site only targets that transcript.

(2) Prioritize functional domains

If the catalytic domain, DNA-binding domain, membrane localization domain, or protein interaction region of the target protein is known, sgRNAs can be preferentially placed in these key regions. In this way, even if the indel is not in the first exon, it is more likely to cause obvious functional loss.

(3) Cross-validation with multiple sgRNAs

A single sgRNA may have insufficient efficiency or off-target risk. In functional studies, 2–3 independent sgRNAs are usually recommended, and whether they produce consistent phenotypes should be observed. If multiple sgRNAs point to the same conclusion and rescue experiments can restore the phenotype, the reliability of the result is higher.

3.2 sgRNA Design in Knock-In and Point Mutation

Knock-in and point mutation experiments depend more on HDR, so the sgRNA cleavage site should be as close as possible to the base to be edited or the insertion site. The farther the cleavage site is, the lower the probability that the donor template will be precisely used. The donor template should also be designed with silent mutations that disrupt the PAM or sgRNA recognition sequence to prevent Cas9 from cleaving again after editing is completed.

(1) Donor template matching the editing goal

Small point mutations or short tag insertions can use ssODN donors. Larger tag inserts, selection markers, or reporter gene insertions usually require double-stranded DNA or plasmid donors. Donor template length, homology arm design, and mutation position all affect HDR efficiency.

(2) Positive clones require genotyping

Knock-in experiments should not only detect whether a PCR band appears. The insertion orientation, junction sequence, copy number, and unedited allele status also need to be confirmed. For models requiring biallelic editing, monoallelic, biallelic, and random integration events should be further distinguished.

3.3 Common Reasons for sgRNA Design Failure

sgRNA design failure does not necessarily appear as a complete absence of editing. It may also appear as low editing ratio, unchanged protein expression, inconsistent phenotype, or large differences between clones. Common reasons include the target site not covering major transcripts, indels not causing frameshift, long half-life of the target protein, compensatory responses to target gene loss, or highly similar off-target sites in the genome.

Table 3 sgRNA design parameters and experimental impact

Parameter	Design requirement	Experimental impact
PAM position	A recognizable PAM must exist near the target sequence	Determines whether Cas9 can cleave the target region
Exon selection	Prefer common exons or coding regions of key functional domains	Affects whether knockout covers the major protein product
GC content	Avoid excessively high or low GC content	Affects sgRNA stability and targeting efficiency
Target distance	For knock-in, the target should be as close as possible to the mutation or insertion site	Affects HDR-mediated precise editing efficiency
Transcript coverage	Consider major transcripts and alternative splicing	Avoid residual functional protein
Off-target prediction	Exclude highly similar genomic sequences	Reduces non-target site editing risk
sgRNA number	Design multiple independent sgRNAs for each gene	Improves reliability of functional conclusions

4 Selection of Cas9 Form and Delivery System

4.1 Cas9 Expression Forms

Cas9 can enter cells in the form of plasmid, mRNA, RNP, or viral vector. The core differences among these forms lie in expression duration, delivery efficiency, cytotoxicity, and off-target risk.

(1) Plasmid system

Plasmid systems are convenient to construct and can simultaneously express Cas9 and sgRNA. They are suitable for routine adherent cells and condition optimization. Their limitation is longer expression duration, which may increase off-target risk. For difficult-to-transfect cells, plasmid entry efficiency may also limit editing efficiency.

(2) Cas9 mRNA

Cas9 mRNA can be transiently expressed in cells, reducing long-term Cas9 exposure. This method is suitable for experimental systems in which continuous Cas9 expression is undesirable, but it has higher requirements for RNA quality, delivery conditions, and cell status.

(3) Cas9 RNP

Cas9 protein and sgRNA are preassembled into an RNP complex before direct delivery. The RNP format acts rapidly and persists for a short time, and is commonly used for primary cells, immune cells, or editing experiments requiring better off-target control.

(4) Viral vector

Lentivirus is suitable for difficult-to-transfect cells and pooled screening, while AAV is commonly used for donor templates or in vivo delivery. Viral systems require attention to vector capacity, expression persistence, integration risk, and biosafety requirements.

4.2 Selection Logic for Different Delivery Systems

The delivery system should be selected according to cell type and experimental purpose. Easily transfected cells can first use plasmid or RNP. Suspension cells and primary cells often require electroporation of RNP. When stable expression or library screening is needed, lentiviral systems are more commonly used. For precise knock-in, the delivery method must also consider the entry efficiency of Cas9, sgRNA, and donor template simultaneously.

Table 4 Cas9 delivery forms and applicable scenarios

Delivery form	Advantages	Limitations	Applicable scenarios
Plasmid	Easy to operate and suitable for routine construction	Long expression duration; low efficiency in some cells	Cell line knockout and initial condition optimization
Cas9 mRNA	Transient expression and reduced prolonged exposure	High RNA stability requirements	Short-term editing and avoidance of long-term expression
Cas9 RNP	Rapid action and relatively low off-target risk	Requires optimization of electroporation or delivery conditions	Primary cells, immune cells, high-precision editing
Lentivirus	Suitable for stable expression and library screening	Integration risk	Pooled screening and difficult-to-transfect cells
AAV	Suitable for some in vivo and donor delivery applications	Limited vector capacity	Small Cas9, HDR donor, or in vivo models

5 Editing Result Validation and Interpretation

5.1 DNA-Level Validation

DNA-level validation is used to confirm whether editing has occurred at the target site. Common methods include PCR, Sanger sequencing, T7E1/Surveyor assays, amplicon deep sequencing, and junction PCR. For pooled cell populations, mixed Sanger peaks can only provide preliminary judgment. For single-clone cells, the mutation type of each allele needs to be clarified.

(1) Gene knockout validation

Knockout experiments should confirm whether the indel causes a frameshift, premature termination, or disruption of a key functional domain. If the indel is a multiple of 3 bp, it may only cause deletion of a few amino acids and may not necessarily inactivate the protein.

(2) Knock-in validation

Knock-in experiments need to validate the 5' junction, 3' junction, inserted fragment sequence, and potential random integration. If the goal is endogenous tag knock-in, it should also be confirmed that the tag is in the correct reading frame and does not disrupt protein localization or function.

5.2 RNA- and Protein-Level Validation

The presence of DNA editing does not necessarily mean that gene function is completely altered. Some mutations may be bypassed by alternative splicing or may produce truncated proteins. qPCR can detect changes in mRNA level, but it cannot fully replace protein validation. Western blot, immunofluorescence, flow cytometry, or tag antibody detection is more suitable for confirming protein loss, localization change, or knock-in tag expression.

5.3 Functional-Level Validation

CRISPR/Cas9 is ultimately used to answer gene function questions, so functional validation is central to result interpretation. Different research directions may select cell proliferation, apoptosis, migration, invasion, drug sensitivity, reporter gene, metabolic flux, or signaling pathway assays. Functional results should be interpreted together with negative sgRNA, positive controls, multiple sgRNA repeats, and rescue experiments to avoid mistaking clonal differences or off-target effects for target gene function.

Table 5 CRISPR/Cas9 editing result validation strategies

Validation level	Common methods	Main question answered
DNA level	PCR, Sanger sequencing, amplicon sequencing, junction PCR	Whether the target site has been edited and whether the editing type is correct
RNA level	RT-qPCR, RNA-seq	Whether target gene transcription level has changed
Protein level	Western blot, immunofluorescence, flow cytometry, tag antibody detection	Whether the protein is lost, reduced, mislocalized, or correctly knocked in
Phenotype level	Proliferation, apoptosis, migration, drug sensitivity, reporter gene assays	Whether editing leads to interpretable functional changes
Causal validation	Rescue, multiple sgRNA repeats, off-target site sequencing	Whether the phenotype is caused by target gene alteration

6 Off-Target Control and Failure Troubleshooting

6.1 Off-Target Effect Control

Off-target effects arise from partial matching of sgRNA to non-target sequences, prolonged Cas9 expression, or complex cellular genomic backgrounds. Off-target risk control should start from the design stage rather than being remedied only after abnormal results appear.

(1) Control during the design stage

During sgRNA design, highly similar off-target sites should be excluded, especially potential off-target sites located in coding regions, promoter regions, or near functional genes. For mechanistic studies or subsequent animal model construction, highly specific sgRNAs should be prioritized.

(2) Control during the delivery stage

RNP or mRNA delivery usually makes it easier to control Cas9 exposure time than continuously expressed plasmids. If a long-term lentiviral expression system is used, result interpretation should be strengthened through multiple sgRNAs, consistent phenotypes, and rescue experiments.

(3) Control during the validation stage

Key conclusions require support from off-target site sequencing or amplicon sequencing. If multiple independent sgRNAs all produce consistent phenotypes and sgRNA-resistant rescue restores the phenotype, the likelihood of off-target explanations is greatly reduced.

6.2 Common Failure Causes and Optimization Strategies

CRISPR/Cas9 experimental failure is often not caused by a single factor, but by the combined effects of target design, delivery efficiency, cell status, repair pathway, and screening strategy. During troubleshooting, the first step should be determining which stage the problem occurs in, rather than repeatedly replacing a single reagent.

Table 6 Common problems and optimization directions in CRISPR/Cas9 experiments

Problem	Possible cause	Optimization direction
Low editing efficiency	Low sgRNA activity, poor delivery efficiency, poor cell status	Replace sgRNA, optimize transfection/electroporation conditions, detect Cas9 expression
Incomplete knockout	Indel does not cause frameshift, long protein half-life, transcript not covered	Screen single clones, reselect common exons, detect protein level
Low knock-in efficiency	Weak HDR activity, poor donor template design, cleavage site too far away	Optimize donor, synchronize cell cycle, shorten distance to cleavage site
Severe cell death	Delivery conditions too strong, Cas9 or antibiotic selection pressure too high	Reduce delivery intensity, optimize antibiotic kill curve
Inconsistent phenotype	Clonal differences, off-target effects, compensatory mechanisms	Validate with multiple sgRNAs, perform rescue, increase independent clones
High NHEJ background	Competition between HDR and NHEJ	Use HDR enhancement or NHEJ inhibition strategies and evaluate toxicity
Confusing sequencing results	Mixed population sample, impure clone	Perform single cloning, amplicon sequencing, or clone sequencing

7 Application Scenarios and Experimental Strategy Selection

7.1 Disease Model Construction

CRISPR/Cas9 can be used to construct models related to cancer, genetic diseases, immune diseases, and metabolic diseases. Gene knockout is suitable for studying loss of function, point mutation is suitable for mimicking disease mutations, and tag knock-in is suitable for observing endogenous protein localization. After model construction, allele status, clonal background, and adaptive changes caused by long-term culture should be considered.

7.2 Drug Target Validation

In drug development, CRISPR/Cas9 can be used to verify whether candidate targets affect cell proliferation, apoptosis, drug sensitivity, or signaling pathways. Single-gene editing is suitable for validating defined targets, while library screening is suitable for discovering resistance factors, synthetic lethal genes, and pathway dependencies.

7.3 Reporter Systems and Endogenous Tag Construction

Through HDR, fluorescent proteins, epitope tags, or reporter genes can be inserted into endogenous loci to observe protein localization, transcriptional activation, or pathway responses under conditions close to physiological expression levels. This strategy is closer to the real regulatory background than exogenous overexpression, but it requires higher HDR efficiency, clone screening, and tag function validation.

7.4 Functional Gene Screening

CRISPR library screening can systematically identify functional genes at the whole-genome or pathway-specific level. Screening quality depends on library coverage, multiplicity of infection, cell number, selection pressure, sequencing depth, and statistical analysis. Screening results cannot be directly used as final mechanistic conclusions and need to be further validated through single-gene sgRNAs, independent cell models, and functional experiments.

8 Related Reagent and Material Selection

Table 7 CRISPR/Cas9 editing core components, vector systems, and phenotype validation materials

Application module	Cat. No.	Product Name	Grade/Specification	Application Positioning
Cas9 nuclease	C744421	Cas9 NLS (SpCas9-NLS)	Animal Free, Carrier Free, EnzymoPure™, RNase free, sterile, 20μM (~3.2μg/μL)	Forms RNP complexes with sgRNA for targeted DNA cleavage and gene knockout
Cas9 nuclease	C744422	Recombinant Cas9 Nuclease (SpCas9)	Animal Free,Carrier Free,EnzymoPure™,RNase free,sterile,Recombinant,1μM (158ng/μL )	Suitable for in vitro RNP assembly, cell editing, and condition optimization
Cas9 nuclease	S1446678	SaCas9		Small Cas9 system suitable for different PAM requirements or vector capacity-limited scenarios
Cas9 mRNA	L1427848	LZCap AG(3'Acm) Cas9 mRNA		Short-term Cas9 expression, suitable for reducing sustained expression and potential off-target risk
Cas9 activity regulation	C1427850	Cas9-IN-1	≥98%	Research on Cas9 activity regulation and editing window control
Cas9 activity regulation	C1427842	Cas9-IN-2		Cas9 inhibition-related mechanism research and editing process regulation
Cas9 activity regulation	C1497202	Cas9-IN-3	Moligand™, 10 mM in DMSO	Cas9 inhibitor stock solution format, suitable for condition screening in cellular systems
Cas9 activity regulation	C1427851	Cas9-IN-3	≥99%	Cas9 activity inhibition and off-target risk control-related research
Cas9 detection	Ab097741	Recombinant CRISPR-Cas9 Antibody	Recombinant, ExactAb™, Validated, See COA	Detection of Cas9 protein expression for Western blot or immunodetection
sgRNA synthesis	O1373182	One-Step sgRNA Synthesis Kit	BioReagent, for RNA applications	In vitro synthesis of sgRNA, suitable for RNP editing and rapid target validation
Lentiviral vector	P749121	pLenti-U6-gRNA-Cas9-P2A-EGFP-Bla		Simultaneous expression of gRNA, Cas9, and EGFP for Bla selection systems
Lentiviral vector	P749122	pLenti-U6-gRNA-Cas9-P2A-EGFP-Hygro		Simultaneous expression of gRNA, Cas9, and EGFP for Hygro stable selection
Lentiviral vector	P749123	pLenti-U6-gRNA-Cas9-P2A-EGFP-Neo		Simultaneous expression of gRNA, Cas9, and EGFP for Neo/G418 selection
Lentiviral vector	P749124	pLenti-U6-gRNA-Cas9-P2A-EGFP-Puro&Zeocin		EGFP marker with Puro/Zeocin dual-selection vector, suitable for stable edited cell enrichment
Lentiviral vector	P749125	pLenti-U6-gRNA-Cas9-P2A-EGFP-Zeocin		Simultaneous expression of gRNA, Cas9, and EGFP for Zeocin selection systems
Lentiviral vector	P749126	pLenti-U6-gRNA-Cas9-P2A-mCherry-Bla		mCherry-labeled Cas9/gRNA lentiviral vector for Bla selection and fluorescence tracking
Lentiviral vector	P749127	pLenti-U6-gRNA-Cas9-P2A-mCherry-Hygro		mCherry-labeled Cas9/gRNA lentiviral vector for Hygro selection
Lentiviral vector	P749128	pLenti-U6-gRNA-Cas9-P2A-mCherry-Neo		mCherry-labeled Cas9/gRNA lentiviral vector for Neo/G418 selection
Lentiviral vector	P749129	pLenti-U6-gRNA-Cas9-P2A-mCherry-Puro&Zeocin		mCherry marker with Puro/Zeocin dual-selection vector, suitable for stable edited cell enrichment
Lentiviral vector	P749130	pLenti-U6-gRNA-Cas9-P2A-mCherry-Zeocin		mCherry-labeled Cas9/gRNA lentiviral vector for Zeocin selection systems
Control sample	C744976	Control Knockout HEK293T RIPA Lysate	BioReagent,for western blot	Western blot control material in gene knockout validation
Cell proliferation detection	C1507077	CFDA SE Cell Proliferation Assay and Tracking Kit	BioReagent	Detects cell division, proliferation, and tracing changes after editing
EdU detection	E1373491	EdU Cell Proliferation Detection Kit (AF488)	Bioactive, Biological Stain, for microscopy, for fluorescence analysis	Suitable for DNA synthesis detection in the green fluorescence channel
EdU detection	E1373492	EdU Cell Proliferation Detection Kit (AF594)	BioReagent,Biological Stain,for microscopy,for fluorescence analysis	Suitable for proliferation detection in the red fluorescence channel
EdU detection	E1373493	EdU Cell Proliferation Detection Kit (AF647)	BioReagent,Biological Stain,for microscopy,for fluorescence analysis	Suitable for proliferation detection in far-red channels or multicolor experiments
Cell viability detection	M405849	MTT Solution [for Cell proliferation assay]	5.0 mg / mL in PBS	Detection of cell viability and proliferation changes after editing
Cell proliferation/cytotoxicity detection	C1375225	MTT Cell Proliferation and Cytotoxicity Assay Kit	BioReagent	Suitable for evaluating cell viability after gene knockout or knock-in
Cell proliferation/cytotoxicity detection	W1507451	WST-1 Cell Proliferation Assay Kit	BioReagent,sterile-filtered,for detection	Water-soluble tetrazolium salt assay for detecting viable cell proliferation and toxicity
Cell proliferation/cytotoxicity detection	C1505852	XTT Cell Proliferation and Cytotoxicity Assay Kit	BioReagent	Detects cellular metabolic activity and cytotoxicity after editing
EdU detection	A598376	Aladdin ® 555 click it edu universal cell proliferation detection kit (orange red fluorescence)		Used for proliferation phenotype validation after gene editing
Apoptosis detection	A1456535	Annexin V- AF488/PI Apoptosis Detection Kit	Suitable for Immunofluorescence(IF),BioReagent,Biological Stain,for microscopy,ready-to-use	Detects early and late apoptosis changes after editing
Apoptosis detection	A1456539	Annexin V- AF647/7-AAD Apoptosis Detection Kit	BioReagent, Biological Stain, for microscopy, Suitable for Immunofluorescence(IF), ready-to-use	Suitable for far-red apoptosis detection and multicolor experiments
Apoptosis detection	A1456530	Annexin V-FITC/7-AAD Apoptosis Detection Kit	BioReagent, Biological Stain, for microscopy, Suitable for Immunofluorescence(IF), ready-to-use	Suitable for routine FITC-channel apoptosis detection
Apoptosis detection	A1372402	Annexin V-PE/7-AAD Apoptosis Detection Kit	BioReagent, Biological Stain, for microscopy, Suitable for Immunofluorescence(IF), ready-to-use	Suitable for apoptosis phenotype analysis in the PE channel
TUNEL detection	T1456509	Colorimetric TUNEL Apoptosis Assay Kit	BioReagent,Colorimetry,for microscopy	Colorimetric detection of DNA breaks and apoptotic cells
TUNEL detection	A598363	Aladdin ® 488 TUNEL apoptosis Kit (green fluorescence)		Fluorescence detection of DNA breaks and apoptotic cells after editing
TUNEL detection	A598362	Aladdin ® 594 TUNEL apoptosis Kit (red fluorescence)		Suitable for DNA break detection in the red fluorescence channel
TUNEL detection	A598360	Aladdin ® 640 TUNEL apoptosis Kit (far red fluorescence)		Suitable for far-red channels or multiplex apoptosis detection
Cell cycle/apoptosis detection	P1373478	Cell Cycle and Apoptosis Analysis Kit	BioReagent, for microscopy, Biological Stain, Suitable for Immunofluorescence(IF), for fluorescence analysis, for cell culture, ready-to-use, for DNA and RNA applications, sterile-filtered	Simultaneous evaluation of cell cycle distribution and apoptosis changes after editing
Cell cycle/apoptosis detection	C1372320	Cell Cycle and Apoptosis Analysis Kit	BioReagent, Suitable for molecular biology	Used for functional phenotype validation and cell cycle analysis in edited cells

9 Frequently Asked Questions

9.1 Does CRISPR/Cas9 knockout always completely eliminate protein expression?

Not necessarily. Indels may not cause frameshifts, or they may produce truncated proteins or retain partial domains. Knockout experiments should combine target-site sequencing, protein detection, and functional readouts for confirmation.

9.2 Can pooled edited cells be used directly for functional experiments?

Pooled edited cells are suitable for rapid preliminary screening, but they often contain a mixture of wild-type cells, heterozygous mutants, biallelic mutants, and different indel types. If stable mechanistic conclusions are needed, single clones should be screened or multiple sgRNAs should be used for cross-validation.

9.3 Why does the same gene require multiple sgRNAs?

Multiple sgRNAs can reduce misinterpretation caused by insufficient efficiency or off-target effects of a single target site. If different sgRNAs produce consistent phenotypes and rescue experiments restore the phenotype, the conclusion is more reliable.

9.4 What are the main reasons for low knock-in efficiency?

Common reasons include the cleavage site being far from the target position, unreasonable donor template design, low HDR activity, poor cell status, or strong NHEJ competition. Knock-in experiments usually require optimization of the donor, delivery method, and screening strategy.

9.5 How can one determine whether a phenotype comes from the target gene rather than off-target effects?

Multiple independent sgRNAs, predicted off-target site sequencing, protein validation, and rescue experiments should be combined. For key mechanistic conclusions, results from a single sgRNA are usually insufficient to support causal interpretation.

9.6 How should Cas9 RNP, plasmid, and lentiviral systems be selected?

RNP is suitable for short-term editing, primary cells, and experiments requiring low off-target risk. Plasmids are suitable for routine cell lines and condition optimization. Lentiviral systems are suitable for difficult-to-transfect cells, stable expression, and library screening. Selection should consider delivery efficiency, expression duration, cytotoxicity, and experimental purpose.

The core of a CRISPR/Cas9 experiment is connecting editing design, molecular validation, and functional validation. Only when target design, delivery method, editing genotyping, off-target control, and phenotype validation are all clearly established can gene editing results be translated into reliable functional conclusions.

For more related articles, please see below:

[1] CRISPR/Cas9 system-based gene knockout technology in animals