Activating mutations in mouse Il-7r, which are homologous to two IL-7R mutations identified in human ETP-ALL (IL7R-241-242TC and IL7R-GCinsL243),58 result in an aggressive ETP-like disease in mice.86 Transduced thymocytes expressing these mutations stop maturing at the double negative 2 (DN2, CD4–CD8–) stage of T cell differentiation ex vivo compared with thymocytes transduced with wild-type Il-7r and/or empty vector. The gene discussed is CD4; the disease is acute lymphoblastic leukemia.