The MCM2-7 complex, which may act as a replicative helicase during DNA synthesis, plays a central role in S-phase genome stability. MCM proteins are required for processive DNA replication and are a target of S-phase checkpoints. Loss of MCM function causes DNA damage and genome instability. MCM expression is upregulated in proliferating cells, providing a diagnostic marker for both cancerous cells and cells with the potential to become malignant. The role of the MCM complex in genome integrity reflects its activity both at active replication forks and away from forks.