Regulation of RNA polymerase II (RNAPII) activity is an essential process that governs gene expression, however its contribution to the fundamental process of erythropoiesis remains unclear. HEXIM1 regulates RNAPII activity by controlling the location and activity of pTEFb (positive transcription factor beta). We identified a key role for HEXIM1 in controlling erythroid gene expression and function, with overexpression of HEXIM1 promoting erythroid proliferation and fetal globin …