Production of XMRV SU fusion protein and its binding to cells Soluble XMRV SU and JSRV SU fusion proteins were produced as described previously [43] . 293T cells were transfected using the calcium-phosphate method with plasmids encoding the different SU. Twelve hours post-transfection, media were replaced with DMEM supplemented with 2% ultra-low IgG FBS (Invitrogen). The proteins in the media were purified using protein A beads (GE Healthcare, Uppsala, Sweden) and analyzed by SDS-PAGE and Sypro Ruby staining (Bio-Rad, Hercules, CA).