Cyclin B (CCNB)

Cyclin B1 is a regulatory protein involved in mitosis. The gene product complexes with p34(cdc2) to form the maturation-promoting factor (MPF). Two alternative transcripts have been found, a constitutively expressed transcript and a cell cycle-regulated transcript, that is expressed predominantly during G2/M phase of the cell cycle. The different transcripts result from the use of alternate transcription initiation sites. Cyclin B1 contributes to the switch-like all or none behavior of the cell in deciding to commit to mitosis. Its activation is well regulated, and positive feedback loops ensure that once the cyclin B1-Cdk1 complex is activated it is not deactivated. Cyclin B1-Cdk1 is involved in the early events of mitosis such as chromosome condensation, nuclear envelope breakdown, and spindle pole assembly.