Serine/threonine-protein kinase that plays a critical role in the control of the eukaryotic cell cycle; involved in G0-G1 and G1-S cell cycle transitions. Interacts with CCNC/cyclin-C during interphase. Phosphorylates histone H1, ATF1, RB1 and CABLES1. ATF1 phosphorylation triggers ATF1 transactivation and transcriptional activities, and promotes cell proliferation and transformation. CDK3/cyclin-C mediated RB1 phosphorylation is required for G0-G1 transition. Promotes G1-S transition probably by contributing to the activation of E2F1, E2F2 and E2F3 in a RB1-independent manner. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with CABLES1 and ATF1. Binding to CCNC/cyclin-C promotes RB1 phosphorylation (By similarity). Binds to CABLES2. Expressed in heart, lung, nasal cavity, roof-plate of the fourth ventricle, skin, cartilage primordium of the basisphenoid and basioccipital bone at 12.5 dpc. A point mutation converts Trp-187 to a premature termination codon, resulting in a truncated form that has no catalytic activity. This truncation occurs near the T loop, which is involved in activation by CDK-activating kinase and deletes motif XI known to be required for kinase function. Consequently, the truncated protein generates a null allele. This mutation is found in laboratory strains but not in wild-mice species such as M.spretus and M.musculus castaneus. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily. Defined as a polymorphic pseudogene by MGI.