Reciprocal regulation of Wee1 and Cdk1. The feedback loop is a double-negative one, resulting in positive regulation of cyclin B/Cdk1 as the activity of this Cdk complex rises. Wee1 inhibits Cdk1 by phosphorylating it on tyrosine 15. Myt1 also performs this modification, though Wee1 appears to be dominant. Cdc25 phosphatases (A, B, C) remove the phosphate group. Cdk1 can also phosphorylate Wee1, inhibiting it.