In the early 1980's Alec Jeffreys found that an individual's hypervariable DNA pattern is so special that only two people in perhaps hundreds of millions have the same one. DNA fingerprinting is important, because although conventional blood tests have become increasingly sophisticated in identifying individuals, the same cannot be said for semen samples. Traditional methods often cannot determine more than blood type from semen stains. The new approach uses DNA from blood, skin, hair, or semen. Sample fragments are combined with radioactive bits of other DNA so that each hypervariable sector leaves a print of dark bands on film. The sequence of these bands, which shows how the DNA's four component proteins are arranged, is the fingerprint. Although DNA fingerprinting may be used in murder or rape cases, its most widespread application is in parenthood and immigration cases. Advantages of DNA fingerprinting are that it is still reliable when drawn from decomposed samples and from samples as old as 4 years. Compulsory large-scale testing of possible crime suspects or other persons might be unconstitutional in the United States. A suggestion that convicted sex offenders provide blood samples as a condition for parole could provoke objections on such grounds.