MCL1 and neoplasm: Evidence of CDK9 inhibition based on RNAP II P-Ser2 and MCL1 downregulation was apparent across all 7 xenograft tumor models (Figure S5) even though measuring RNAP II P-Ser2 at the 6 hr time point used for this study, focusing on apoptotic response, was slightly late for those early markers based on kinetic studies described above (Figure 5A).