Absence of XMRV/MuLV sequences by real-time PCR in PBMC DNA of persons with and without CFS from the US. Representative real-time XMRV polymerase (pol) PCR results. Upper panel; pol amplification plot using XMRV synthetic DNA diluted in a background of 2.5 ug of DNA from whole blood to 12,000, 1,200, 120 and 12 copies and negative (water and DNA) controls demonstrating the sensitivity and dynamic linear range of the assay. Lower panel; pol amplification plot for DNA from 40 persons, including 18 with severe CFS, 8 with unclassified CFS, and 14 without CFS. Two positive controls (DNA from 17 XMRV infected 22Rv1 cells spiked into 2.5 μg of human leukocyte DNA) for the pol PCR, and two negative controls (2.5 μg of DNA) are also shown. Only the two positive controls were detected in this testing.