CDKN1A and cystic fibrosis: Results of p53 and p21 expression quantified by ELISA showed no statistically significant differences in p53 protein expression in T21F versus CF, whereas p21 protein expression significantly increased with in vitro replicative aging in T21F (30%, P < 0.05) and in control fibroblasts (35%, P < 0.05) (Figure 8(c)).