For validation studies, the specific CDK9 inhibitor NVP-2 (Fig. 3A) and the chemically distinct inhibitor AZD4573 (Fig. 3B) known to induce tumor regression in a PDX model of AML (11) and now being examined in phase II trials for advanced hematological malignancies (NCT03263637, NCT04630756) were used. Here, CDK9 is linked to acute myeloid leukemia.