Genomic masking of nondefective recombinant murine leukemia virus in Moloney virus stocks

HIX virus cloned from Moloney leukemia virus stocks is a nondefective, leukemogenic, and amphotropic murine oncornavirus with a recombinant-type major glycoprotein. Although Moloney leukemia virus stocks generally contain little or no free amphotropic virus, dilution analysis of several virus stocks and the examination of virus progeny from individual foci revealed that HIX virus is present and functionally coated with ecotropic Moloney virus envelopes. Because most mice have serum factors that inactivate recombinant viruses, masking may represent a general survival mechanism for HIX as well as other analogous recombinant viruses.