Cyclin-dependent kinase 7 (CDK7) has essential roles in the cell-division cycle as a CDK-activating kinase (CAK) and in the transcription as a component of the general transcription factor TFIIH. This protein forms a trimeric complex with cyclin H and MAT1, which functions as a Cdk-activating kinase (CAK), phosphorylating cell-cycle CDKs. It is also an essential component of the transcription factor TFIIH, which phosphorylates the C-terminal domain (CTD) of the largest subunit of Pol II. This protein is thought to serve as a direct link between the regulation of transcription and the cell cycle. Embryonic stem cells (ESCs) shows a very unusual cell cycle structure, characterized by a short G1 phase and a high proportion of cells in S-phase. This is associated with a unique mechanism of cell cycle regulation by the activity of cyclin dependent protein kinase (Cdk). The unique cell cycle structure and mechanism of cell cycle control indicates that the cell cycle machinery plays a role in establishment or maintenance of the stem cell state. Cyclin-dependent kinases (CDKs) are activated by the binding to a cyclin and mediate the progression through the cell cycle. Each different complex controls a specific transition between two subsequent phases in the cell cycle. CDK7 is the catalytic subunit of the CDK-activating kinase (CAK) complex, a serine-threonine kinase. CAK activates the cyclin-associated kinases CDC2/CDK1, CDK2, CDK4 and CDK6 by threonine phosphorylation. CAK complexed 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. Involved in cell cycle control and in RNA transcription by RNA polymerase II. Its expression and activity are constant throughout the cell cycle.