In 1986, Sir Alec Jeffreys broke up cells and extracted their DNA. He added radioactive probes and left it in a tank over the weekend.
He then checked on them that Monday, and used Restriction Fragment Length Polymorphism to analyze the DNA. He discovered that Variable Number of Tandem Repeats were present, and each person's was a different length.
He realized that the variety in length could identify a specific individual in a technique he named "genetic fingerprinting".
Then, in Leicestershire, Dawn Ashworth was murdered and raped three years after the same thing happened to Lynda Mann.
Jeffreys tested the blood type and enzymes of the killer, narrowing down the suspect list. He tested over four thousand men, but one man in particular was avoiding a blood test.
The man avoiding being tested was Colin Pitchfork. Pitchfork was tested and his DNA matched perfectly with the DNA of the muderer. He pled guilty to both rapes and murders.