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. Here, IL15 is linked to leukemia.