Cyclin-dependent kinase 7 (Cdk7) is the catalytic subunit of the CDK-activating kinase (CAK) comple, Xenopus/Amphibian,, a serine-threonine kinase. Cdk7 activates the cyclin-associated kinases CDK1, CDK2, CDK4 and CDK6 by threonine phosphorylation. Cdk7 comple, Xenopus/Amphibian,ed to the core-TFIIH basal transcription factor activates RNA polymerase II by serine phosphorylation of the repetitive C-terminus domain (CTD) of its large subunit (POLR2A), allowing its escape from the promoter and elongation of the transcripts.