Isolation and characterization of a unique C57BL B-tropic virus

Abstract A unique B-tropic virus (CS43) has been isolated from a C57BL mouse. This virus differs from those we have previously characterized and from several isolates we have obtained from normal and leukemic C57BL mice. Unlike other C57BL B-tropic viruses, CS43 does not have a p12 polypeptide homologous to that of the class II xenotropic virus, but does have a p12 homologous to that of AKR MuLV (class I). In common with the other C57BL B-tropic isolates, CS43 has an altered major core protein (p30), which competes only weakly in a class-specific assay that distinguishes ecotropic MuLV p30s from those of xenotropic origin. The data are compatible with a unique recombinational event in the generation of this isolate, which should prove useful in studying the origin of B-tropic viruses and the properties that determine N/B tropism.