Cdk1: the dominant sibling of Cdk2

Cdk2 is thought to regulate entry into S phase, whereas Cdk1 controls the initiation of mitosis. New evidence shows that Cdk1 is equally capable of promoting the G1/S transition in Cdk2−/− cells, raising the question as to whether Cdk1 constitutes the predominant cyclin-dependent kinase in mammalian cells, or only compensates for Cdk2 function when it is compromised.