To conduct a PCR analysis, lab workers distill DNA from a suspect's blood and from bodily material collected at the crime scene (a minute amount of skin, blood, hair, saliva, or semen will do). From each sample, they select 13 to 16 specific segments of DNA and amplify them with PCR. The copies of each segment are deposited in ultrathin test tubes that are attached to a computerized instrument. The instrument generates a positive charge, and the negatively charged DNA travels toward it, with larger bits of DNA moving more slowly. The computer then calculates the size of each DNA segment. The probability that the selected DNA segments from one person will register as precisely the same size as corresponding segments from an unrelated person is less than one in 10 billion.