Involvement of p19ARF in the sustained induction of p21 after IR. Asynchronous populations of wild-type (WT) and p19ARF−/− MEFs were treated with 6 Gy of γ-radiation and harvested after 2, 4, 6, 8, 10, 24, or 48 h. For p53 and p21 analysis, 50 μg of protein was resolved on an SDS/10% polyacrylamide gel. For analysis of p19ARF, 100 μg of protein was resolved on a 12% polyacrylamide gel. (A) p53, p21, and actin protein levels in WT and p19ARF−/− MEFs 2 h (T2), 4 h (T4), 6 h (T6), 8 h (T8), 10 h (T10), 24 h (T24), and 48 h (T48) after IR compared with untreated (Unt). (B) p19ARF and actin protein levels in WT MEFs at the indicated times after IR compared with untreated (Unt). p53−/− and p19ARF−/− MEFs are also shown as positive and negative controls for p19ARF protein detection.