MPF is activated at the end of G2 by a phosphatase, which removes an inhibitory phosphate group added earlier.
To the contrary, MPF is also present in somatic cells, where it induces entry into M phase of the mitotic cycle.
Evidence that a diffusible factor regulates the entry into mitosis had been previously obtained in 1966 using the slime mold Physarum polycephalum in which the nuclei of the multi-nucleate plasmodial form undergo synchronous mitoses.
This result demonstrated that mitotic entry was controlled by a diffusible cytoplasmic factor and not by a "nuclear clock.
The CDK1 subunit is now free and can bind to cyclin B, activate MPF, and make the cell enter mitosis.
MPF is disassembled when anaphase-promoting complex (APC) polyubiquitinates cyclin B, marking it for degradation in a negative feedback loop.