 Since the action of cyclin A/CDK2 inhibits that of cyclin E/CDK2, the sequential activation of cyclin E followed by the activation of cyclin A is important and tightly regulated in S phase.In late S phase, cyclin A can also associate with CDK1.Cyclin A remains associated with CDK1 from late S into late G2 phase when it is replaced by cyclin B. Cyclin A/CDK1 is thought to be involved in the activation and stabilization of cyclin B/CDK1 complex.