If you were investigating a crime scene, you wouldn’t just accuse the nearest bystander. The real culprit could be miles away.

In 2007, a team of British researchers announced that genetic variants within the FTO gene could predispose people to being fat. On average, people with one set of these variants weighed 1.6 kilograms more than people with none, and those with two sets—including one in six Europeans—weighed 3 kilograms more.

It was an important discovery. By studying twins, scientists had already shown that obesity runs in families, to an extent that can’t be explained by a shared environment. It was clear that some genes can influence how much we weigh (though, obviously, not exclusively*), but no one had identified any of them. FTO was the first. The media, as is their unfortunate wont, labelled it a “fat gene”.

Many later studies corroborated FTO’s connection to body weight. When scientists deleted the gene in mice, the rodents grew up thinner. When they switched the gene on, over and above its usual activity, mice ate more and put on weight. The pieces seemed to fit.

But right from the start, FTO was shrouded in mystery. No one really knew what the gene did, nor how it could influence our weight. And some parts of the FTO story just didn’t make sense.