CK1 (casein kinase 1) is a group of serine/threonine protein kinases that are involved in the regulation of various cellular processes, including cell growth, differentiation, and metabolism. CK1 kinases are characterized by a conserved catalytic domain that is involved in the phosphorylation of downstream signaling molecules. There are several different types of CK1 kinases, each with a specific substrate specificity and function.