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). The activity of this complex is regulated by binding with 7SK snRNA (By similarity). Plays a role during muscle differentiation; P-TEFB complex interacts with MYOD1; this tripartite complex promotes the transcriptional activity of MYOD1 through its CDK9-mediated phosphorylation and binds the chromatin of promoters and enhancers of muscle-specific genes; this event correlates with hyperphosphorylation of the CTD domain of RNA pol II (PubMed:16245309, PubMed:23060074, PubMed:12037670). In addition, enhances MYOD1-dependent transcription through interaction with PKN1 (By similarity). Involved in early embryo development (PubMed:19364821). Interacts with CDK9 to form P-TEFb. Interacts with POLR2A (via the C-terminal domain (CTD)); mediates transcriptional activity. Interacts with HEXIM1; mediates formation of a tripartite complex with KPNA2. Interacts with HEXIM2. Interacts with PKN1; enhances MYOD1-dependent transcription. P-TEFB complex interacts with RB1; promotes phosphorylation of RB1 (By similarity). P-TEFB complex interacts with MYOD1; promotes the transcriptional activity of MYOD1 through its CDK9-mediated phosphorylation (PubMed:12037670). Interacts with MDFI and MDFIC (By similarity). Nucleus in differentiating cells. Highly expressed in all phases of skeletal muscle differentiation, particularly in later stages (PubMed:23060074). Highly expressed in skeletal muscle. Significantly expressed in heart, brain, kidney, liver, testis, and pancreas (PubMed:19364821). Expressed in all forming organs and supporting tissues in 10.5 to 14.5 dpc. Expressed in embryonic ectoderm and the forming brain and neural tube in 7.5 and 8.5/9.5 dpc embryos, respectively. The homozygous knockout of Ccnt2 is embryonic lethal. Belongs to the cyclin family. Cyclin C subfamily.