Insertion of a single long terminal repeat (LTR) of an intracisternal A particle (IAP) gene into a placenta-expressed cellular gene (mouse IAP-promoted placental gene, MIPP) has recently been found for the mouse strains Swiss and CF-1. To test whether such a retrotransposition event has also taken place in other strains of mouse, genomic DNA and total RNAs were analyzed from the outbred CF-1 strain and five inbred strains of mouse, AKR/J, C57BL/6J, DBA/2J, C3H/HeJ, and BALB/c. Specific DNA fragments flanking the LTR region were isolated from MIPP cDNA and genomic clones and used as hybridization probes. Three transcripts [two minor, 4.4 kilobases (kb) and 2.2 kb; one major, 1.2 kb] were detected. The 4.4-kb and 2.2-kb species were found in all strains of mouse studied. The 1.2-kb transcript (promoted by IAP LTR) is present only in placentas of C3H/HeJ and BALB/c mice. Both of these strains contain an IAP LTR in the MIPP gene. In contrast, there is no IAP LTR in this cellular gene in strains DBA/2J, C57BL/6J, and AKR/J. Thus, the IAP MIPP retrotransposition is strain-specific. The parents of the outbred CF-1 mice used for the present studies were both heterozygous at the MIPP locus. Restriction fragment length polymorphism studies using inbred strains and recombinant inbred mice have further established the linkage between the MIPP gene (D4Jhu8) and several loci on distal mouse chromosome 4. The symbol Ipp is being used for this gene in all major mouse data bases and in the comparative section of the human genome data base.