Analysis of established primary cancer cells stained with Giemsa and May-Grunwald stains indicated that while both RRM1 and RRM2 similarly impacted growth in colony assays (78 colonies for MSI2-HOXA9, ~35 colonies for both ∆RRM1 and ∆RRM2; Fig. 2B, Fig. S1E), only deletion of RRM1 impacted differentiation (Fig. 2F-I, Fig. S1F-H), leading to a reduction in blast cell counts (~80% for MSI2-HOXA9, 60% for BCR-ABL only, 58% for ∆RRM1; Fig. 2F) and an increase in differentiated myeloid cells (Fig. 2G-I), and resulting in a disease similar in composition to that driven by BCR-ABL alone. The gene discussed is RRM2; the disease is cancer.