ATM and lymphoma: Given the predominance of lymphoma formation in Atm−/−Smg1gt/+ mice, we examined whether the addition of Smg1 heterozygosity resulted in increased defects in the immune system compared with the known decrease in T cells caused by loss of ATM expression.2, 3 We showed previously that SMG1 heterozygosity alone did not significantly alter the composition of immune system prior to disease onset.12 Here we analysed the composition of circulating blood cells and lymphocytes in the spleen, thymus and lymph nodes.