|  | heterodimeric protein that initiates prophase of mitosis 
                        and alters the activity of diverse proteins that function 
                        in mitosis by phosphorylation; active MPF contains both 
                        cyclin B (large) and cyclin-dependent kinase 1 (small) 
                        subunits. Activity is regulated partly by the synthesis 
                        and degradation of cyclin B subunit. |  |