Seems to phosphorylate critical substrates that regulate the G1/S phase transition and/or DNA replication. Can phosphorylate MCM2 and MCM3. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Forms a complex with either DBF4/DBF4A or DBF4B, leading to the activation of the kinase activity. Additional isoforms seem to exist. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDC7 subfamily.