English: This diagram illustrates the feedback loops underlying the G2/M transition. Cyclin-B1/CDK1 activates Plk and inactivates Wee1 and Myt1. Activated Plk activates cdc25. Activation of cdc25 and inactivation of Wee1/Myt1 lead to further activation of Cyclin-B1/CDK1. Also shown is the putative role of cyclin-A/CDK2 as an initial activator of the feedback loop, discussed in a later section.