Protein kinases are enzymes that transfer a phosphate group from a phosphate donor, generally the g phosphate of ATP, onto an acceptor amino acid in a substrate protein. By this basic mechanism, protein kinases mediate most of the signal transduction in eukaryotic cells, regulating cellular metabolism, transcription, cell cycle progression, cytoskeletal rearrangement and cell movement, apoptosis, and differentiation. With more than 500 gene products, the protein kinase family is one of the largest families of proteins in eukaryotes. The family has been classified in 8 major groups based on sequence comparison of their tyrosine (PTK) or serine/threonine (STK) kinase catalytic domains. The casein kinase 1 (CK1) group consists of 12 kinases including CK1, TTBK (tau tubulin kinase), and VRK (vaccinia-related kinase) families. The receptor guanylate cyclase (RGC) group consists of 5 kinases similar in domain sequence to TKs (ANP, CYG).