Homologous recombination occurring among misaligned repeated sequences is a significant source of the molecular rearrangements resulting in human genetic disease. Studies of the Charcot-Marie-Tooth disease locus on chromosome 17 have implicated the involvement of an ancient DNA transposon of the mariner family (Hsmar2) in the initiation of double-strand break events leading to homologous recombination. In this study, the genomic locations of 109 Hsmar2 elements were determined by primed in situ labeling (PRINS) using primers designed to match the right and left inverted terminal repeats (ITRs) of the transposon. Although the resolution of the PRINS technique is approximately 400 chromosomal Giemsa bands, the data presented here provide the first large-scale mapping study of these elements, which may be involved in initiation of homologous recombination events in the human genome.