Cyclin-dependent kinase 7 (CDK7) is a member of the CDK family of protein kinases that regulate cell cycle progression. CDK7 is the catalytic subunit in a trimeric complex with cyclin H and MAT1 that functions as a Cdk-activating kinase (CAK). CDK7 activates the kinase activities of CDK1, CDK2, CDK4, and CDK6 by threonine phosphorylation. In addition to its cell cycle function CDK7 has also been found to be an essential component of the transcription factor TFIIH. Several substrates of CDK7 are involved in transcription, further indicating a role in transcription.