Fig. 7. Models of Cdc25A regulation and function in the mammalian cell cycle. (A) A dual positive feedback loop leading to maximal and irreversible activation of the M-phase-promoting cyclin B–Cdk1 kinase. See Discussion for details. (B) Phosphorylation-dependent switches among three differentially stable forms of Cdc25A, designated labile, ultra-labile and stable, determine the thresholds and roles of Cdc25A in unperturbed or DNA-damaged interphase and mitotic cells, respectively. See Discussion for details.