Cortisol is a hormone produced by the adrenal glands in times of stress, and it also plays many roles in your normal bodily functions. It is a multi-purpose hormone that needs to be in the right amount (not too high, not too low) and at the right time. Your genes play a big role in how likely you are to have problems with cortisol.
The initial research on the FTO gene was all about how it relates to increased BMI and increased fat cell creation. More recently, researchers discovered that FTO is an m6A eraser that removes methyl groups from mRNA. This discovery has opened up huge avenues of research on topics from cancer to immune response to heart disease.