IL7R and precursor B-cell acute lymphoblastic leukemia: To more rigorously evaluate the capacity of mutant IL-7Rα to transform B cell precursors in vivo and to originate B-ALL, we first introduced a human type 1a mutant IL7R form7,13 into the mouse coding sequence and validated its ability to promote constitutive signaling and to transform Ba/F3 cells (Supplementary Fig. 1).