Cyclins are the regulatory subunits of Cdc2 p34 and related cyclin-dependent kinases (Cdks) which play critical roles in the control of cell cycle progression. The catalytic subunit for cyclin A and B is Cdc2 p34 kinase. The Cdc2- cyclin B complex controls the G2 to M transition whereas Cdc2-cyclin A regulates S phase progression. The G1 to S transition, however, appears to be controlled by the G1 cyclins. Cyclin D1 accumulates during G1 and associates with Cdk2, Cdk4 and Cdk5. Cyclin E and Cdk2 interact during the G1 to S transition. Cyclin G contains a typical N terminal cyclin box and a carboxy terminal domain sequence homologous to the tyrosine phosphorylation site of the epidermal growth factor receptor. Cyclin G expression is induced within 3 hours after growth stimulation and remains elevated with no apparent cell cycle dependency. Cyclin G2 shares 53% amino acid sequence identity with cyclin G1. Peak expression of cyclin G2 is seen in late S phase, as opposed to cyclin G1 expression, which is constitutive.