RUNX1 and chronic myelogenous leukemia, BCR-ABL1 positive: LoF mutations, especially those targeting DNA binding, displayed higher fitness with larger cell counts, suggesting that RUNX1 acts as a tumor suppressor in this setting of CML with blast crisis, and its loss provides a selective growth advantage, although it is important to note that K562 cells represent disease that developed on a WT RUNX1 background.