CAK (also known as CDK-activating kinase, CDK7 and cell division protein kinase 7 is a 40 kD nuclear protein that is a member of the serine/threonine family of protein kinases, CDC2/CDKX subfamily. CAK activates cyclin-associated kinases CDC2/CDK1, CDK2, CDK4, CDK6 by threonine phosphorylation and controls cell cycle progression, DNA repair, and RNA polymerase II transcription. CAK is activated by phosphorylation at Thr170 and is inactivated by phosphorylation at Ser164. CAK has been reported to interact with cyclin H and MAT1 to form the CAK complex, the CAK complex then interacts with core-TFIIH to form the TFIIH basal transcription factor. The Poly6324 antibody is useful for immunofluorescence microscopy of human CAK protein.