M-MuLV glyco-gag enhances XMRV infectivity. A) Viruses were gathered from stably infected 293 T cells and measured for infectivity by the focal immunofluorescence assay (FIA) in HeLa cells. Infectivities were normalized by the amount of Capsid (CA) in each stock and shown relative to wild-type XMRV. B) pVP62 was co-transfected with different M-MuLV glyco-gag expression vectors into 293 T cells and viruses gathered from the supernatants were titered in HeLa cells by FIA. Infectivities were normalized for the amount of CA, and the infectivities relative to control XMRV (co-transfection with empty expression vector) are shown. Welch’s t-test with unequal variances was conducted to detect the difference between mean scores in the two groups and values significantly different from XMRV (A) or pXMRV control (B) are indicated (*, p < 0.05. **, p < 0.01).