New studies have indicated that the cell membrane receptor for the ecotropic murine leukemia virus is the classic membrane amino acid transporter y+, the principal transporter of cationic L-acids in mammalian cells. This finding has been hailed as a landmark in cell physiology which may reveal new mechanisms of viral pathogenesis. These studies represent the first amino acid transporter to be cloned, as well as the first example of a virus subverting a transmembrane protein as a receptor.