Cellular and whole-organism viability is impaired in ATM and TERT doubly null mice. (A) Survival curves for mice of the indicated genotypes (left). ATM^−/−TERT^−/− mice had shorter longevity than ATM^−/− mice. Data shown are survival rates expressed as a percentage. Results were analyzed with a log-rank nonparametric test (right). (B) SA-β-gal staining of frozen sections of thymus and spleen taken from mice of the indicated genotypes (12 weeks of age). Tissues taken from ATM^−/−TERT^−/− mice demonstrated increased SA β-gal staining, compared with other littermates. (C) Immunohistochemical analysis against p16, p21, and γH2AX in thymus and spleen from mice of the indicated genotypes (36 weeks of age). While p16 is increased in sections from TERT^−/−, ATM^−/−, and ATM^−/−TERT^−/− mice, p21 is decreased in these sections. γH2AX is also increased, especially in the ATM^−/− and ATM^−/−TERT^−/− thymus and spleen.