Screening of a 129/J mouse genomic library under nonstringent hybridization conditions with a xenotropic virus-like long terminal repeat (LTR) probe revealed a family of sequences resembling insertion elements (IS) with structural features of solitary retroviral LTRs; these are called LTR-IS. They are interspersed among variable flanking regions of mouse DNA and lack any viral structural genes. LTR-IS elements start and end with 11-base-pair inverted repeats and contain signals implicated in RNA polymerase II transcriptional regulation: C-C-A-A-T, T-A-T-A-A-A, and A-A-T-A-A-A. The members of the family are homologous, but not identical, approximately equal to 500-base-pair-long elements with 4-base-pair target-site duplications on both sites of the element. There are 500 LTR-IS per mouse haploid genome.