In contrast to the studies mentioned above, XMRV was not found in PC and CFS patient cohorts from CKI-7 dihydrochloride several European and US studies. Studies of the prevalence of XMRV in two PC patient cohorts in Germany found, for example, no link between prostate cancer and the presence of XMRV when DNA or RNA from tumor samples was analyzed. Also, analyses of CFS cohorts from England and Netherlands failed to detect XMRV using PCR analysis. Likewise, an ELISA-based screen of antibodies in plasma of PC patients detected no XMRV-specific responses and no antibodies against XMRV were found in sera of CFS patients when XMRV pseudoviruses were used in a neutralization assay. In a study from the Centers for Disease Control and Prevention, there was no evidence of XMRV infection in 50 CFS patients or 56 healthy controls. Some have speculated that geographical restrictions account for the differences in detecting XMRV; however, the fact that the assays and reagents varied among the studies described above may also have BMS-870145 contributed to the differences in findings. Thus, additional investigations are needed to sort out those discrepancies and reveal the true prevalence of XMRV infection. In our recent study of XMRV serological prevalence in a cohort of PC patients, we observed approximately 25% positivity for serum XMRV antibodies ; however, despite this relatively high incidence, the XMRV antibody titers were low overall compared to those of HIV-1 infected individuals. To provide an explanation for the low magnitude of immune responses observed in our PC cohort, we initiated a study of XMRV immune responses in a murine model. We hypothesized that low immunogenicity is an inherent characteristic of an XMRV infection. To test this hypothesis, we vaccinated mice to elucidate the magnitude and duration of the antibody response against the XMRV Env antigen. An HIV-1 pseudovirus-based assay has been widely used for the detection of NAb in sera from HIV-1 infected patients and experimentally infected/vaccinated animal models. We therefore adapted the assay using an XMRV pseudovirus to determine the utility of such an approach for detecting XMRV NAbs. The infectivities of the XMRV and control HIV-1 pseudoviruses were compared by monitoring the levels of bgalactosidase expression in TZM-BL cells after 48 hours of infection. The results indicated that the XMRV pseudovirus is,250 times more infectious than the control HIV-1 pseudovirus.