To test the role of activated IL7RA in leukemia initiation, we expressed wild type and/or an activated mutant form of human IL7RA containing an in-frame insertion (PPCL—p.Leu243_Thr244insProProCysLeu) (IL7RAins)18 in human umbilical cord blood (CB) hematopoietic progenitors. The gene discussed is IL7R; the disease is leukemia.