Synergistic activity of combined CDK9/BET inhibition in MLL-r PDXs in vitro. PDX cells from 4 MLL-ALL (n = 2) (A) and 1 MLL-AML (n = 3) (B) were treated for 48 hours, with single treatment of JQ1 (blue), CDKI-73 (light green), or the combination (red). Viability was measured by annexin V and 7-aminoactinomycin D (7AAD) staining and flow cytometry analysis. The double-negative (annexin V and 7-AAD negative) population (viable cells) of treated samples is plotted as a percentage of the no drug control (NDC). Bliss-additive effect is indicated by the purple dashed lines. Viability below this curve indicates synergy. IC₅₀, 50% inhibitory concentration.