Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. Can phosphorylate casein, phosvitin, myosin light chains and poly(Glu,Tyr) in vitro. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Partially inhibited by N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide (CKI-7). Monomer. Autophosphorylated. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.