This technique permits specific short regions of a gene to be greatly amplified in vitro from as little as a single molecule of DNA. Through this technique, genetically variable mitochondrial and nuclear DNA sequences have been detected from the root region of shed hair as well as from freshly-plucked, single hairs. These sequences have also been detected in a sample from a single hair shaft. Three methods of DNA typing have been used: the determination of amplified DNA fragment length differences, hybridization with allele-specific oligonucleotide probes, and direct DNA sequencing. These approaches differ from the most common method of DNA analysis, restriction fragment length polymorphism, which requires much larger samples than can be obtained from most forensic samples, such as single hairs and bloodstains or from anthropological, genetic, or zoological samples collected in the field. However, all the DNA identification techniques have produced significant advances in gene and disease mapping and in the forensic identification of individuals. Figures and 35 references. (Author abstract modified)