Hexamethylene bisacetamide-inducible proteins (HEXIMs) are transcriptional regulators that function as general RNA polymerase II transcription inhibitors. HEXIMs, aided by the 7SK snRNA, sequester positive transcriptional elongation factor b (P-TEFb) into a large inactive 7SK snRNP complex [, ]. This prevents P-TEFb from phosphorylating RNA polymerase II and blocks subsequent transcriptional elongation.Two HEXIM family members have been identified in mammals, termed HEXIM1 and HEXIM2, which exhibit distinct expression patterns []. In HEXIM1-knocked down cells, HEXIM2 can functionally and quantitatively compensate for the loss of HEXIM1 to maintain a constant level of the 7SK/HEXIM-bound P-TEFb [].
