Analyses of thymocytes in NOD.Scid.Il15−/− mice prior to T-ALL development revealed discernible changes within the CD4−CD8− double-negative (DN) thymocyte developmental stages and increased frequencies of CD4+CD8+ double-positive cells with a high proportion of TCR-negative CD4+ and CD8+ cells. The gene discussed is IL15; the disease is acute lymphoblastic leukemia.