Cyclin H belongs to the highly conserved cyclin family, and are regulators of CDK kinases. Different cyclins exhibit distinct expression and degradation patterns which contribute to the temporal coordination of each mitotic event. Cyclin H forms a complex with CDK7 kinase and ring finger protein MAT1. The kinase complex is able to phosphorylate CDK2 and CDC2 kinases, and thus functions as a CDK-activating kinase (CAK). Cyclin H and its kinase partner are components of TFIIH, as well as RNA polymerase II protein complexes.