The RT-PCR products were analyzed using the Ibis T5000 universal biosensor platform (Ibis Biosciences, Inc., Carlsbad, CA; http://www.ibisbiosciences.com), which performs automated post-PCR desalting, ESI-MS signal acquisition, spectral analysis, and data reporting as described previously [23] . Briefly, the steps were as follows: 15 mL aliquots of each PCR reaction were desalted and purified using a weak anion exchange protocol as described elsewhere [25] . Accurate mass (61 ppm), high-resolution (M/dM.100,000 FWHM) mass spectra were acquired for each sample using high-throughput ESI-MS protocols described previously [20] . For each sample, approximately 1.5 mL of analyte solution was consumed during the 74-second spectral acquisition. Raw mass spectra were post-calibrated with an internal mass standard and deconvolved to monoisotopic molecular masses. Unambiguous base compositions were derived from the exact mass measurements of the complementary single-stranded oligonucleotides [26] . Quantitative results are obtained by comparing the peak heights with an internal PCR calibration standard present in every PCR well at 100 molecules [20] .