 The amount of cyclin B (which binds to Cdk1) and the activity of the cyclin B-Cdk complex rise through the cell cycle until mitosis, where they fall abruptly due to degradation of cyclin B (Cdk1 is constitutively present).The complex of Cdk and cyclin B is called maturation promoting factor or mitosis promoting factor (MPF).Cyclin B is necessary for the progression of the cells into and out of M phase of the cell cycle.