Northern blot analysis of RNR expression in stable 3T3 cell pools transfected with either pCaggs empty vector or pCaggs RNR genes. Total RNA was extracted from the indicated cell lines and subjected to Northern blot hybridization with probes specific for Rrm1, Rrm2, p53R2, or Gapdh. (B) Western blot analysis of RNR protein expression in RNR overexpressing 3T3 cells. Total protein was extracted from the indicated cell lines and subjected to immunoblotting with antibodies specific to Rrm1, Rrm2, or p53R2. Duplicate membranes were immunoblotted for β-actin as a loading control. Samples in (A) and (B) were run on single blots, which were then cropped to remove extraneous lanes. (C) Mutation frequency at the Hprt locus in Rrm1, Rrm2 and p53R2 overexpressing 3T3 cells. Mutation frequency was determined by Hprt assay.