HEXIM1 and neoplasm: Upregulation of HEXIM1 has been identified as a potential pharmacodynamic biomarker in numerous tumour xenografts and whole blood samples in response to ABBV-075 treatment (Fig. 2).109 HEXIM1 inhibits P-TEFb by sequestering it into an inactive complex; accordingly, upregulation of HEXIM1 leads to the inhibition of transcriptional elongation by RNA polymerase II.110 This represents an indirect mechanism through which BETi could repress transcription.