A new gamma retrovirus, Xenotropic Murine leukaemia Virus-related virus (XMRV), was identified in 2006 and its association was claimed with prostate cancer (PC) and chronic fatigue syndrome (CFS). A series of studies from disparate geographical areas have failed to substantiate these claims. Recent studies have suggested that XMRV may have arisen in mice through recombination of two proviruses .
Regardless of the controversies, XMRV is a culturable virus capable of infecting different cell types like T and B Lymphocytes, NK cells, etc., . Intravenous inoculation of Rhesus Macaques with XMRV showed organ-specific cell tropism, infecting CD4 T cells in lymphoid organs including the gastrointestinal lamina propria, alveolar macrophages in lung, and epithelial/interstitial cells in other organs, and cells of the reproductive tract . Many retroviruses are pathogenic (HIV-1) causing severe disease but at the same time they have been used in gene therapy which requires targeting of the virus to the host cell through interactions between viral envelope proteins and cell surface proteins. It is important to determine the mode of its cell entry to define tropism, and understand virus transmission and pathogenesis. Biological process involves highly specific interactions and infection of cells by viruses is no exception. Such interactions are of major importance in pathogenic viruses as they are potential drug targets. One such classic example is the entry of HIV-1 through a series of interactions between the viral gp120 and cellular receptor CD4 and co-receptor such as either CCR5 or CXCR5 .
The xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs) all rely on the XPR1 receptor for entry, but these viruses vary in tropism, distribution among wild and laboratory mice, pathogenicity, strategies used for transmission, and sensitivity to host restriction factors . XMRV is closely related to xenotropic murine leukemia viruses MLVs (X-MLVs) . The X-MLVs and polytropic MLVs (P-MLV) use Xpr1 as a receptor for cell entry [4, 7, 8], and so does XMRV [9–11]. The recent identification of MLV and XMRV in human prostate cancer tissues, peripheral blood mononuclear cells (PBMCs) of chronic fatigue syndrome patients, and the respiratory tract of immunocompromised patients  raises the concern of a potential threat to public health from cross-species transmission of MLV related viruses.