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) (PubMed:9499409, PubMed:15563843). The activity of this complex is regulated by binding with 7SK snRNA (PubMed:11713533). 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 (By similarity). In addition, enhances MYOD1-dependent transcription through interaction with PKN1 (PubMed:16331689). Involved in early embryo development (By similarity). (Microbial infection) Promotes transcriptional activation of early and late herpes simplex virus 1/HHV-1 promoters. Interacts with CDK9 to form P-TEFb (PubMed:9499409, PubMed:16331689). Interacts with POLR2A (via the C-terminal domain (CTD)); mediates transcriptional activity (PubMed:15563843). Interacts with HEXIM1; mediates formation of a tripartite complex with KPNA2. Interacts with HEXIM2 (PubMed:19883659). Interacts with PKN1; enhances MYOD1-dependent transcription (PubMed:16331689). P-TEFB complex interacts with RB1; promotes phosphorylation of RB1 (PubMed:12037672). P-TEFB complex interacts with MYOD1; promotes the transcriptional activity of MYOD1 through its CDK9-mediated phosphorylation (By similarity). Interacts with MDFI and MDFIC (PubMed:17289077). Interacts with MON1B; down-regulates CCNT2-mediated activation of viral promoters during herpes simplex virus 1/HHV-1 infection (PubMed:21509660). (Microbial infection) Interacts with HIV-2 and SIV Tat. Does not bind efficiently to the transactivation domain of the HIV-1 Tat (PubMed:10364329). Nucleus in differentiating cells. Ubiquitously expressed. Belongs to the cyclin family. Cyclin C subfamily.