To demonstrate a match, a lab typically looks at 13 places within the genetic code of the DNA. That code consists of long sequences of chemicals along the DNA molecule, somewhat like the sequences of letters in a message. In this case, the message is written with a four-letter alphabet.

In the 13 places, the code tends to stutter with a four-letter repeat, and the number of repeats varies widely between people. The lab counts the number of repeats in each of the 13 places, and the overall pattern is a person's DNA profile. In fact, each place can yield two numbers, because people inherit DNA from both parents, and the number of repeats inherited from each parent usually differs.