I have always wondered why DNA is an accepted evidence. It's so easy to contaminate a crime scene or bring someone else hair, skin cells, etc by mistake.
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"Mr Butler has a rare skin condition, which means he sheds flakes of skin, leaving behind much larger traces of DNA than the average person. He worked as a taxi driver, and so it was possible for his DNA to be transferred from his taxi via money or another person, onto the murder victim."