Cyclin B1 is a regulatory protein involved in mitosis. The gene product complexes with p34 (Cdk1) 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. Once activated, cyclin B1-Cdk1 promotes several of the events of early mitosis. The active complex phosphorylates and activates 13S condensin, which helps to condense chromosomes. Another important function of the cyclin B1-Cdk1 complex is to break down the nuclear envelope. The nuclear envelope is a membranous structure containing large protein complexes supported by a network of nuclear lamins. Phosphorylation of the lamins by cyclin B1-Cdk1 causes them to dissociate, compromising the structural integrity of the nuclear envelope so that it breaks down. The destruction of the nuclear envelope is important because it allows the mitotic spindle to access the chromosomes.