XMRV SU binding assays were performed as described previously [6, 43] . Cells were incubated with 2-10 mg of soluble XMRV SU-IgG fusion protein for 3-4 h on ice, washed 3 times with PBS-2% FBS and incubated with anti-human IgG coupled to FITC for detection. Fluorescence was measured by flow cytometry using FACSCalibur (BD Bioscience, Missauga, Canada).