IFN-γ enhances CR mRNA levels in U937 cells as determined by standardized RT-PCR. Samples containing similar a amount of viable cells as determined by trypan blue exclusion were analyzed at days 2 and 5 after treatment with IFN-γ. To determine the increase in mRNA expression upon treatment, samples were amplified using different dilutions (as indicated) and β-globin amplification was used to confirm comparability of the samples. The influence of possible contamination by genomic DNA in RT-PCR reactions was excluded by control amplifications in the absence of RT, because no signals were obtained under these conditions (not shown). Results are representative of 3 independent experiments.