round spermatids from 26-month-old control mice showed a significantly elevated spontaneous mutant frequency (1.26 ± 0.35 × 10−5) relative to those from 4-month-old (0.41 ± 0.18 ×10−5,p=0.032) and 15-month-old mice (0.32 ± 0.18 × 10−5, p=0.032) (Table 1)