UV-induced effect on the expression of P21 mRNA in MRC5 cell line. MRC5 cells were exposed to UV at 20 J/m2 and harvested 0, 1, and 3 hr after the treatment as indicated. Total RNA was extracted and RT-PCR was performed. The ratio of P21 mRNA to G3PDH is shown. Data represent the means ± SD of three independent experiments.