[Mitotic cyclins--new possibilities for examining mechanisms of neoplasm growth].

Mitotic cyclins constitute a regulatory subunit of the histone H1 kinase complex. On the ground of their primary structure they are divided into two classes A and B, both necessary for the mitosis. Cyclin A activates histone H1 kinase and becomes destroyed by proteolysis earlier than cyclines B and plays an important role in the DNA replication. Cyclins A and B may be involved in the development of neoplastic disorders either directly (inappropriate expression of the cyclin A gene caused by hepatitis B virus in hepatocellular carcinoma, or interactions of this cyclin with factors participating in the regulation of cell proliferation) or indirectly by phosphorylation of some oncogene or antioncogene proteins by a cdk (cyclin dependent kinase).