Transcriptional regulator which functions as a general RNA polymerase II transcription inhibitor (PubMed:14580347, PubMed:15713661, PubMed:15201869). 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:12832472, PubMed:14580347, PubMed:15713661, PubMed:15201869). May also regulate NF-kappa-B, ESR1, NR3C1 and CIITA-dependent transcriptional activity (PubMed:15940264, PubMed:15941832, PubMed:17088550). Plays a role in the regulation of DNA virus-mediated innate immune response by assembling into the HDP-RNP complex, a complex that serves as a platform for IRF3 phosphorylation and subsequent innate immune response activation through the cGAS-STING pathway (PubMed:28712728). Homooligomer and heterooligomer with HEXIM2; probably dimeric (PubMed:15713661, PubMed:15965233, PubMed:16377779). 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:12832472, PubMed:15201869, PubMed:15169877, PubMed:16362050, PubMed:15713661, PubMed:15855166, PubMed:16377779, PubMed:17643375, PubMed:17724342). Interacts with the N-CoR complex through NCOR1 (PubMed:17452463). Interacts with ESR1 and NR3C1 (PubMed:15940264, PubMed:15941832). May interact with NF-kappa-B through RELA (PubMed:12581153). Interacts with CCNT2; mediates formation of a tripartite complex with KPNA2 (PubMed:19883659). Part of the HDP-RNP complex composed of at least HEXIM1, PRKDC, XRCC5, XRCC6, paraspeckle proteins (SFPQ, NONO, PSPC1, RBM14, and MATR3) and NEAT1 non-coding RNA (PubMed:28712728). Binds alpha-importin and is mostly nuclear (PubMed:16362050). Ubiquitously expressed with higher expression in placenta. HEXIM1 and HEXIM2 are differentially expressed. Expressed in endocrine tissues. Up-regulated by HMBA (hexamethylene bisacetamide) (at protein level). Down-regulated by estrogen. The coiled-coil domain mediates oligomerization. Inhibits Tat activity which is required for HIV-1 transcription. Belongs to the HEXIM family.