UVB induces expression of Id3 in immortalized keratinocytes. Cells were irradiated with 480 J/m2 of UVB and then harvested at 4 h (a) or the indicated time points (b, c). Id3 mRNA steady state levels were quantified by RT-PCR and electrophoresis (a) or Q-PCR (b). (b) Fold expression of Id3/cyclophilin mRNA levels for each time point compared to unirradiated (Time=0) controls are shown and presented as mean ± S.D. of three replicates of a representative experiment; essentially the same results were obtained in three independent experiments. One, two, or three asterisks denote significance levels (compared to control) of p<.05, p<.01; or p<.001, respectively. (c) Id3 proteins levels were determined by immunoblot analysis following UVB irradiation.