CHEK1 and leukemia: Collectively, our results show that Runx1/RUNX1 loss reduces sensitivity to CHK1 inhibitors across systems: Srsf2P95H and U2AF1S34F mouse progenitors, SRSF2-mutant patient samples with RUNX1 mutation, and human leukemia lines with SRSF2 mutation (K562, KO52).