In certain mouse cells transformed by polyomavirus (Py), viral DNA is excised from the chromosome together with a defined mouse component, which has been designated Ins [Bourgaux et al., Virology 122 (1982) 84-97]. Ins carries a sequence which anneals with the DNA at many sites in the mouse genome, while displaying no detectable homology with well-characterized SINEs and LINEs [Sylla et al., Gene 29 (1984) 343-350]. Recently, others have reported that this sequence belongs to a newly discovered family of repetitive mouse DNA, designated MT for Mouse Transcript [Heinlein et al., Nucl. Acids Res. 14 (1986) 6403-6416]. We demonstrate here that the MT family consists of short interspersed repetitive sequences (SINEs) with structural features of retroposons. Using cloned fragments of Ins as probes, we have identified recombinants carrying MT sequences in a genomic library of mouse DNA. Regions of homology to the probes were subcloned twice from the DNA of lambda phage using plasmids pAT153 and pUC13, and characterized in detail by heteroduplex mapping and by sequencing. The three distinct elements thus studied were highly homologous over 400 bp, terminated in 3' with a short sequence rich in A residues, and were flanked by a short direct repeat. On the basis of these complete elements, a 400-bp consensus sequence was established for the MT family.