still unclear. It is known that cyclin B1 can compensate for loss of both cyclin B2 (and vice versa in Drosophila). Saccharomyces cerevisiae contains six B-type cyclins (Clb1-6), with Clb2 being the most essential for function. In both vertebrates and S. cerevisiae, it is speculated that the presence of multiple B-type cyclins allows different cyclins to regulate different portions of the G2/M transition while also making the transition robust to perturbations. Subsequent discussions will focus on the spatial and temporal activation of cyclin B1/CDK in mammalian cells, but similar pathways are applicable in both other metazoans and in S. cerevisiae. Cyclin B1 synthesis and degradation Cyclin B1 levels are suppressed throughout G1 and S phases by the anaphase-promoting complex (APC), an E3 ubiquitin ligase which targets cyclin B1 for proteolysis. Transcription begins at the end of S phase after DNA replication, in response to phosphorylation of transcription factors such as NF-Y, FoxM1 and B-Myb by upstream G1 and G1/S cyclin-CDK complexes. Regulation of cyclin-B1/CDK1 activity Increased levels of cyclin B1 cause rising levels of cyclin B1-CDK1 complexes throughout G2, but the complex remains inactive prior to the G2/M transition due to inhibitory phosphorylation by the Wee1 and Myt1 kinases. Wee1is localized primarily to the nucleus and acts on the Tyr15 site, while Myt1 is localized to the outer surface of the ER and acts predominantly on the Thr14 site. The effects of Wee1 and Myt1 are counteracted by phosphatases in the cdc25 family, which remove the inhibitory phosphates on CDK1 and thus convert the cyclin B1-CDK1 complex to its fully activated form, MPF. Active cyclinB1-CDK1 phosphorylates and modulates the activity of Wee1 and the Cdc25 isoforms A and C. Specifically, CDK1 phosphorylation inhibits Wee1 kinase activity, activates Cdc25C phosphatase activity via activating the intermediate kinase PLK1, and stabilizes Cdc25A. Thus, CDK1 forms a positive feedback loop with Cdc25 and a double negative feedback loop with Wee1 (essentially a net positive feedback loop). Positive feedback and switch-like activation These positive feedback loops encode a hysteretic bistable switch in CDK1 activity relative to cyclin B1 levels (see figure). This switch is characterized by two distinct stable equilibria over a bistable region of cyclin B1 concentrations. One equilibrium corresponds to interphase and is characterized by inactivity of Cyclin-B1/CDK1 and Cdc25, and a high level of Wee1 and Myt1 activity. The other equilibrium corresponds to M-phase and is characterized by high activity of Cyclin-B1/CDK1