Radiation-induced p53 target gene induction and apoptosis. (a) Impaired IR-induced transactivation of target genes. Using semi-quantitative RT-PCR, induction of Mdm2 and p21 upon γ-irradiation can readily be observed in wild-type and heterozygous embryos, but is absent in animals homozygous for the p53 mutant allele. (b) Suppression of apoptosis in primary cultured cells. Primary cells derived from p53
+/+ embryos were irradiated with 10 Gy of ionizing radiation and observed by time-lapse microscopy. The apoptotic cells from homozygous embryos with fragmented nuclei are indicated with arrows.