MAPK3 and Splenomegaly: The recipient mice which received T315I‐BCR‐ABL+Prmt1fl/fl; Cre‐ERT2 BM cells survived longer (median survival: Prmt1fl/fl vs Prmt1fl/fl; Cre‐ERT2 was 22 vs 75 days; Figure S5B, Supporting Information), showed less severe splenomegaly and spleen weight (Figure S5C,D, Supporting Information) than those which received T315I‐BCR‐ABL+Prmt1fl/fl BM cells.