Inside your cells, you have a set of proteins whose only job is to identify and fix mistakes in your DNA. Every time your cells divide (which, by the way, happens millions of times a day), they have to replicate your genetic code to pass it along. Individual mutations—which is what we call any mistake in your DNA—happen all the time, but your repair proteins fix most of them before they cause any harm. Mutations most often come in the form of one DNA building block being swapped for another, but sometimes they occur because a DNA strand breaks, and the repair protein mends the strand imperfectly. Maybe a piece is lost, or the wrong end of the strand gets stuck back on. If just one strand snaps, the repair proteins can usually match the right end. But if both strands break at once, repair proteins struggle to make it right.