In situ hybridization of hamster/human hybrids with biotinylated human genomic DNA has revealed that human chromosomal DNA can integrate into the hamster genome and is not always cytologically detectable. This finding helps to explain why discordancy can arise in gene mapping by failing to recognize small pieces of foreign DNA in the rodent genome. Fluorescent in situ hybridization allows one to locate these fragments in rodent chromosomes visually and possibly to identify their chromosome of origin.