Note the existence of two
double-negative feedback loops, between CDK1 and Wee1 and between Greatwall kinase and PP2A-B55; these raise in acute form
the question of how the system flips from interphase into M-phase. The return to interphase is easier to understand, because the
inactivation of CDK1 depends on the proteolysis of cyclin, although this process itself is regulated by reversible phosphorylation of
both the APC/C itself, and its activator Cdc20