[Cyclins and proliferative pathology]

Cyclins are proteins which activate a family of serine-threonine kinases called cdk (cyclin-dependent kinases) responsible for progression through the cell cycle. Human cyclins are divided into two groups: mitotic cyclins (cyclins A and B) which regulate cell division, and G1 cyclins (C, D and E) involved in early stages of cell cycle. Cyclins and their cdk partners participate in the pathogenesis of neoplastic proliferation by phosphorylating some oncogenes and antioncogenes, for example the protein encoded by the retinoblastoma susceptibility gene (pRb). Moreover, hyperexpression of one or more classes of cyclins on the gene, mRNA and/or protein level is sometimes observed in neoplastic cells of different origin, for example the rearrangement of the cyclin D1 gene with ectopic expression of its protein product in centrocytic lymphomas with translocation t(11; 14) (q13; q32). Mitotic cyclins can serve as a marker of proliferating cells.