Most of the time when we think about genetic mutations, we think about the ones that spell disaster. But sometimes, our genes can have the opposite effect: Instead of increasing our risk for certain diseases, they can protect us from them.

This has turned out to be the case with a Colombian woman in her 70s. By all indications, she should have developed Alzheimer’s disease by her mid-40s. She has one of three rare mutations that lead to early-onset Alzheimer’s disease—people with this mutation only make up about 1% the 44 million individuals living with Alzheimer’s globally. And yet, by the time she turned 70 she was still mostly healthy.

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The reason for her continued health? A super rare mutation to both copies of another gene related to Alzheimer’s, called APOE. Called the Christchurch mutation (for the town in New Zealand where scientists discovered it in the 1980s), it seems to somehow counteract the risk of Alzheimer’s disease—and could inspire future treatments or preventions for it. Researchers in the US and in Colombia published the woman’s case study on Monday, Nov. 4 in the journal Nature Medicine.

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