In vitro construction of a B-tropic virus by recombination: B-tropism is a cryptic phenotype of xenotropic murine retroviruses.

A B-tropic virus was isolated in vitro from the progeny of mouse cells doubly infected with N-tropic and xenotropic murine leukemia viruses. Biological and structural evidence is presented suggesting that the phenotypically silent structural marker for B-tropism, expressed by the xenotropic virus p30, was transferred to an N-ecotropic virus via recombination, thus resulting in the expression of a B-ecotropic murine leukemia virus.