A51 treatment induced a significant reduction in Pol II CTD phosphorylation at Ser2 or Ser5 in RUNX1 mutant (OCI AML-5) and RUNX1 WT (MV4–11, THP-1, OCI AML-3, and HL-60) AML cell lines (Fig. 4). Here, RUNX1 is linked to acute myeloid leukemia.