UV-induced effect on the expression of P21 mRNA and p21 protein. 293 cells were exposed to UV at 20 J/m2 and harvested 0 to 72 hr after treatment as indicated. Data represent the means ± SD of three independent experiments. (A) The time course of P21 mRNA expression. RNA was extracted for RT-PCR. The ratios of P21 mRNA to G3PDH mRNA were measured with densitometry. (B) The time course of p21 protein expression. Cell lysates were extracted and separated by SDS-PAGE, transferred to a nitrocellulose membrane, and probed with anti-p21 antibody. The signal intensity of the Western blotting is shown.