Transcriptional regulator which functions as a general RNA polymerase II transcription inhibitor (PubMed:15713661, PubMed:15713662). Core component of the 7SK RNP complex: in cooperation with 7SK snRNA sequesters P-TEFb in a large inactive 7SK snRNP complex preventing RNA polymerase II phosphorylation and subsequent transcriptional elongation (PubMed:15713661, PubMed:15713662). Homooligomer and heterooligomer with HEXIM1; probably dimeric (PubMed:15713661, PubMed:15994294). Core component of the 7SK RNP complex, at least composed of 7SK RNA, LARP7, MEPCE, HEXIM1 (or HEXIM2) and P-TEFb (composed of CDK9 and CCNT1/cyclin-T1) (PubMed:15713661, PubMed:15713662, PubMed:15994294). Interacts with CCNT2 (PubMed:19883659). Ubiquitously expressed with higher expression in testis. HEXIM1 and HEXIM2 are differentially expressed. Up-regulated by HMBA (hexamethylene bisacetamide) (at protein level). The coiled-coil domain mediates oligomerization. Belongs to the HEXIM family.