Although the proportion of C57BL/6.scid.Il15−/− mice developing leukemia was only 75% at 8 months of age (Table 1), our findings indicate that the loss of IL-15 signaling promotes the emergence of leukemic cells that show increased NOTCH1 activation from thymocytes harboring the PrkcdScid mutation, irrespective of their genetic background. This evidence concerns the gene NOTCH1 and leukemia.