In our October 2009 publication, we established XMRV infection in the blood products of our patient population by five different methods. Of these methods, single-round PCR on DNA from peripheral blood mononuclear cells (PBMCs), the least sensitive method, required us to use samples from a subset of chronically ill patients we had observed to have persistent viremia. In Figure 1A of our Science paper, we showed that DNA of 7 of 11 patients exhibited the expected gag and env PCR amplification products from single-round PCR with XMRV primers. We included this figure to demonstrate that nested PCR, which inevitably raises questions of contamination, is not essential to detect XMRV in highly viremic ME/CFS patients.