The analysis of the normal expression of endogenous retroviral sequences in congenic 129 GIX+ and GIX- mouse strains suggests the action of a regulatory element, encoded by Gv-1, functioning in trans to control the expression of these sequences. At steady state, the abundance of polyadenylated retroviral transcripts of different length cosegregates with the Gv-1 genotype. The presence of low but detectable quantities of these transcripts in antigen-negative mice indicates however, that Gv-1 may be distinct from the structural genes encoding retroviral proteins. We report here that retroviral transcripts, derived from distinct proviruses, are expressed in a tissue-specific manner and are coordinately regulated by the Gv-1 locus. The structure of several of the abundant transcripts demonstrates further that they are transcribed from independent defective endogenous proviral genomes exhibiting extensive deletions of env coding regions as well as modified U3 regions distinct from those found in exogenous retroviral transcripts.