Transcriptional regulator which functions as a general RNA polymerase II transcription inhibitor. 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. 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. Homooligomer and heterooligomer. Core component of the 7SK RNP complex. Binds alpha-importin and is mostly nuclear. Belongs to the HEXIM family.