In human cells, MCM4, - 6, and - 7 proteins form a stable complex, and MCM2 is loosely associated with this complex. The heterotrimeric MCM complex containing MCM4, - 6 and - 7 ha s DNA helicase activity, and such activity can be inhibited by the presence of MCM2. Cdc7/Dbf4 protein kinase, which is required for the initiation of DNA replication phosphorylates MCM2 in vitro and in vivo. From these observations, it's thought that phos phorylation of MCM2 may remove MCM2 from the MCM4, - 6, and - 7 complex to activate its DNA helicase activity at the onset of DNA replication.