Regulatory subunit of the cyclin-dependent kinase pair (CDK9/cyclin T) complex, also called positive transcription elongation factor B (P-TEFb), which is proposed to facilitate the transition from abortive to production elongation by phosphorylating the CTD (carboxy-terminal domain) of the large subunit of RNA polymerase II (RNAP II). Component of the super elongation complex (SEC), at least composed of Ell, Cdk9, cyclin-T (CycT), lilli and ear. Associates with CDK9 to form P-TEFb. Belongs to the cyclin family. Cyclin C subfamily.