To gain insight into the mechanism of the CHK1-mediated gemcitabine sensitization, we determined mRNA and protein expression levels for the genes RRM1 and RRM2 in the four naturally gemcitabine-resistant NSCLC cell lines and the four in vitro-induced gemcitabine-resistant clones 24 h after transfection. Here, RRM1 is linked to non-small cell lung carcinoma.