Infectivity of XMRV mutants. The wild-type and putative glyco-gag-mutated XMRV stocks (generated by transient transfection of expression vectors in 293 T cells) were used to measure infectivity on HeLa cells by a focal immunofluorescence assay. The amount of viral RNA in virions was determined by quantitative RT-PCR. Both infectivity and the amount of viral RNA were normalized based on Capsid protein in viruses, and the relative infectivities and the amounts of viral RNA relative to wild-type XMRV are shown. Student’s t-test with equal variances was conducted to detect the difference between mean scores in the two groups and values significantly different from wild-type XMRV are indicated (*, p < 0.05, **, p < 0.005, ***, p < 0.0005).