Restoration of RFC gene expression in parental and antifolate-resistant cells upon treatment with pCPT-cGMP. The effect of pCPT-cGMP on the restoration of RFC mRNA levels in parental and antifolate-resistant cells was examined by RT-PCR analysis (30 cycles) using internal GAPDH (glyceraldehyde-3-phosphate dehydrogenase) normalization as detailed in “Materials and methods.”