Cyclin, N-terminal <p>Cyclins are eukaryotic proteins that play an active role in controlling nuclear cell division cycles [<cite idref="PUB00014103"/>], and regulate cyclin dependent kinases (CDKs). Cyclins, together with the p34 (cdc2) or cdk2 kinases, form the Maturation Promoting Factor (MPF). There are two main groups of cyclins, G1/S cyclins, which are essential for the control of the cell cycle at the G1/S (start) transition, and G2/M cyclins, which are essential for the control of the cell cycle at the G2/M (mitosis) transition. G2/M cyclins accumulate steadily during G2 and are abruptly destroyed as cells exit from mitosis (at the end of the M-phase). In most species, there are multiple forms of G1 and G2 cyclins. For example, in vertebrates, there are two G2 cyclins, A and B, and at least three G1 cyclins, C, D, and E.</p><p>Cyclin homologues have been found in various viruses, including <taxon tax_id="10381">Saimiriine herpesvirus 2</taxon> (Herpesvirus saimiri) and <taxon tax_id="37296">Human herpesvirus 8</taxon> (HHV-8) (Kaposi's sarcoma-associated herpesvirus). These viral homologues differ from their cellular counterparts in that the viral proteins have gained new functions and eliminated others to harness the cell and benefit the virus [<cite idref="PUB00014101"/>].</p>Cyclins contain two domains of similar all-alpha fold, of which this entry is associated with the N-terminal domain.