The CK1 family of monomeric serine–threonine protein kinases is found in eukaryotic organisms from yeast to humans. Mammals have seven family members (sometimes referred to as isoforms, but encoded by distinct genes): alpha, beta 1, gamma 1, gamma 2, gamma 3, delta, and epsilon. Isoforms range from 22 to 55 kDa and have been identified in the membranes, nucleus, and cytoplasm of eukaryotes and additionally in the mitotic spindle in mammalian cells. The family members have the highest homology in their kinase domains (53%–98% identical) and differ from most other protein kinases by the presence of the sequence S-I-N instead of A-P-E in kinase domain VIII. The family members appear to have similar substrate specificity in vitro, and substrate selection is thought to be regulated in vivo via subcellular localization and docking sites in specific substrates. One consensus phosphorylation site is S/Tp-X-X-S/T, where S/Tp refers to a phospho-serine or phospho-threonine, X refers to any amino acid, and the underlined residues refer to the target site. Thus, this CKI consensus site requires priming by another kinase. CKI also phosphorylates a related unprimed site, which optimally contains a cluster of acidic amino acids N-terminal to the target S/T including an acidic residue at n − 3 and a hydrophobic region C-terminal to the target S/T. A single acidic residue in the n − 3 position is not sufficient for CKI phosphorylation. In contrast, in several important targets, NF-AT and beta-catenin, CKI does not require n − 3 priming but, instead, phosphorylates the first serine in the sequence S-L-S, which is followed by a cluster of acidic residues, albeit less efficiently than the optimal sites.