['Xenotropic Murine leukemia virus-Related Virus (XMRV) is a γ-retrovirus initially reported to be present within familial human prostate tumors and the blood of patients with chronic fatigue syndrome. Subsequent studies however were unable to replicate these findings, and there is now compelling evidence that the virus evolved through rare retroviral recombination events in human tumor cell lines established through murine xenograft experiments. There is also no direct evidence that XMRV infection has any functional effects that contribute to tumor pathogenesis.', 'Herein we describe an additional xenotropic MLV, "B4rv", found in a cell line derived from xenograft experiments with the human prostate cancer LNCaP cell line. When injected subcutaneously in nude mice, LNCaP cells infected with XMRV or B4rv formed larger tumors that were highly hemorrhagic and displayed poor pericyte/smooth muscle cell (SMC) investment, markers of increased metastatic potential. Conditioned media derived from XMRV- or B4rv-infected LNCaPs, but not an amphotropic MLV control virus infected LNCaPs, profoundly decreased expression of marker genes in cultured SMC, consistent with inhibition of SMC differentiation/maturation. Similar effects were seen with a chimeric virus of the amphotropic MLV control virus containing the XMRV env gene, but not with an XMRV chimeric virus containing the amphotropic MLV env gene. UV-inactivated XMRV and pseudovirions that were pseudotyped with XMRV envelope protein also produce conditioned media that down-regulated SMC marker gene expression in vitro.'] Do xenotropic MLV envelope proteins induce tumor cells to secrete factors that promote the formation of immature blood vessels?