p21 , p16 , Gadd45a and p53 expression in mouse skeletal muscle. (A) Relative expression ratio ± SE for mRNA levels, as determined by RT-PCR, for p21, p16, Gadd45a and p53 in young (5 month old), old (25 month old), and old CR (n = 5 for each group). (B-E) Relative expression ratio ± SE for mRNA levels for these genes over the course of the adult mouse life span (5 to 30 months of age). The mean of 5 mice ± SE was used for each data point on the graphs. Two-tailed Student's t-test; *P < 0.05 (young versus old) and †P < 0.05 (old verses old CR).