These and related data show that the majority of Lck-Cre::Mad2l1f/f::Trp53f/f animals suffered from poorly differentiated CD4+ and CD8+ T-acute lymphoblastic lymphoma (T-ALL) while a subset suffer from more differentiated CD4+ or CD8+ T-ALL as described previously for Trp53null thymic lymphoma (Donehower et al., 1995). The gene discussed is CD4; the disease is acute lymphoblastic leukemia.