People who carry errors in a single gene could be protected against heart attacks and high cholesterol, American research suggests.
The New England Journal of Medicine study reveals one in every 650 people could have genetic faults that halve the chance of suffering heart attacks.
Drugs that work on this gene pathway already exist and researchers hope this sheds light on their effectiveness.
Scientists say their findings may pave the way for improved therapies.
A team of US scientists sequenced the entire genetic code of 22,000 people, identifying 34 people with specific errors. The naturally occurring genetic changes lead to a key gene not activating.
Most people have two active copies of the NPC1L1 gene, but researchers found the 34 people with an inactive copy had lower levels of LDL cholesterol.
The study suggests their cholesterol levels were about 10% lower than those with two active copies of the gene.
And they were at half the risk of suffering heart attacks too.
People with the errors did not differ from the larger population in other ways such as their blood pressure, weight and rate of diabetes - all factors that can contribute to heart attack risk.
Dr Sekar Kathiresan of the Broad Institute, who worked on the study, said: "Protective mutations like the one we've just identified for heart disease are a treasure trove for understanding human biology.
"They can teach us about the underlying causes of disease and point to important drug targets."
Prof Nathan Stitziel of Washington University School of Medicine, who also worked on the research, said: "When people have one copy of a gene not working, it's like taking a drug their entire lives that is inhibiting this gene."
The anti-cholesterol drug Ezetimibe already works by inhibiting the NPC1L1 pathway. But researchers are unsure whether it just lowers cholesterol or directly protects against heart disease too.
Scientists hope this discovery will help answer this question and may spur improved therapies that target this gene.
Prof Jeremy Pearson, from the British Heart Foundation charity, said: "High cholesterol is a major risk factor for coronary heart disease, increasing the risk of a potentially life-threatening heart attack or stroke.
"That's why researchers are working hard to find new ways to treat this condition.
"The results of this genetic study are interesting and strengthen the likelihood that treatment with Ezetimibe, or a more powerful drug acting on the same target, will reduce cardiovascular events.
"Whether Ezetimibe can do so is currently being tested in a large trial, whose results are eagerly awaited."