GammaretrovirusDNA ContaminationEndogenous RetrovirusesRetroviridae InfectionsXenotropic murine leukemia virus-related virusLeukemia Virus, MurineLeukemia Virus, Gibbon ApeReceptors, VirusChiropteraGene Products, gagMoloney murine leukemia virusNIH 3T3 CellsPhylogenyMolecular Sequence DataEvolution, MolecularCell Line