Cyclin H is a member of the cyclin family of regulatory proteins that associate with and activate cyclin-dependent kinases (CDKs) to regulate cell cycle progression. The hallmark characteristic of cyclins is the periodicity of their expression and activity that is regulated transcriptionally and post-transcriptionally via phosphorylation and ubiquitination. Cyclin H associates with CAK [cyclin-dependent activating kinase (CDK) activating kinase], also known as CDK7 or p40MO15. Cyclin H is part of a group of cyclins that are involved in transcriptional regulation. The transcriptional regulating cyclins include cyclin C, H, K, L1, L2, T1, and T2 which associate with CDK7, -8, -9, -10 and -12. The p40MO15(cdk7)/cyclin H/MAT1 complex localizes to the nucleus and is a subunit of the general transcription factor IIH. This complex functions to activate CDKs as well as phosphorylate several components of the transcriptional machinery.