Regulatory component of the TFIIK complex (KIN28-CCL1 dimer) which is the protein kinase component of transcription factor IIH (TFIIH) and phosphorylates the C-terminal domain of RNA polymerase II during transition from transcription to elongation after preinitiation complex (PIC) formation, thereby positively regulating transcription. TFIIH (or factor B) is essential for both basal and activated transcription, and is involved in nucleotide excision repair (NER) of damaged DNA. TFIIH has DNA-dependent ATPase activity and is essential for polymerase II transcription in vitro. CCL1 and KIN28 form the TFIIK complex, a component of TFIIH holo complex. Component of a complex consisting of KIN28, CCL1 and TFB3. Present with 8700 molecules/cell in log phase SD medium. Belongs to the cyclin family. Cyclin C subfamily.