Credit: Martin O’Neill

Go about 145,000,000 bases (or "letters") down the long arm of chromosome 1 and you'll come to 1q21.1, the genetic address of a small but important piece of DNA that is particularly prone to mistakes. When chromosome 1 is duplicated during normal cell division (say, in creating sperm or eggs), short, repetitive bits of DNA within this stretch are all too likely to mistakenly pair up, raising the chances that the new cells will have extra or missing copies of specific pieces of DNA.

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