DNA fragments used as standards in competitive PCR were precisely calibrated using HPLC and commercially available DNA molecular mass markers. The accuracy of calibration was reflected by data that differed by only 2% from the mean when two independently purified and calibrated competitor preparations were compared. Highly dilute competitor solutions were stable at -20 degrees C for up to 1 year in the presence of carrier HindIII-digested lambda DNA, but progressive loss of competitor DNA with increasing storage time was observed when carrier DNA was omitted from the solution. Applying 0.2 U uracil-DNA glycosylase (UDG) per assay of remaining temperature-stable activity did not effect the ratios of synthesized products. This study describes quality management in PCR quantitation that is useful for the measurement of multidrug resistance-associated protein (MRP) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene transcripts.

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of Proc. Natl. Acad. Sci. USA (86,[9717][9718][9719][9720][9721], the authors request that the following changes be noted. Ref. 30, shown below, should be inserted in lines 39 and 42 of the Introduction as follows: "A second approach is to generate an allelic variant (e.g., a small deletion or insertion in the gene of interest) such that there is a small difference in the size of the PCR product of this internal standard and the PCR product of the native mRNA (30). Creation of a restriction enzyme site in the target gene is another method that would permit distinction between the PCR products of the standard and target RNAs (30)."

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