Integration of Moloney leukaemia virus into the germ line of mice: correlation between site of integration and virus activation.

Endogenous avian and murine C-type viruses are genetic elements which are transmitted at several distinct chromosomal loci. Different patterns of virus activation have been observed in a variety of different mouse strains. However, because there are multiple copies of closely related endogenous viruses in every mouse strain, the genetic basis of the differential expression of these genes is poorly understood. A new substrain of mice, BALB/Mo, was derived previously carrying the exogenous Moloney leukaemia virus (M-MuLV) genome on chromosome 6 (ref. 14). Virus activation occurs in lymphatic tissues of all BALB/Mo mice soon after birth. We report here the derivation of three new substrains of mice each carrying a single M-MuLV genome on different chromosomal integration sites. Each locus was associated with a distinct phenotype of virus expression. Evidence is presented that apparently identical viral genomes show differences in spontaneous virus activation and that defective viral genomes can be carried in the germ line of mice.