Lynch syndrome

Lynch syndrome is an inherited condition that makes many kinds of cancer more likely. The highest risks are for colorectal cancer, meaning cancer of the colon and rectum, and endometrial cancer, which affects the endometrium, the layer that lines the inside of the uterus. What gets passed down isn't cancer itself. It's a higher chance of developing it.

To see why, picture what happens inside a cell before it divides. First it copies all of its DNA, the instruction manual the cell runs on, and mistakes slip into that copy: letters that end up mismatched. Fixing them is the job of the mismatch repair genes, such as MLH1 or MSH2, which work like a proofreader checking the copy letter by letter. In Lynch syndrome, one of those genes is inherited in an altered form. In other cases the change is in a different gene, EPCAM, and that change leaves MSH2 switched off. Either way, the proofreader breaks down and the mistakes stay put.

That breakdown leaves a mark on the microsatellites, stretches of DNA where the same sequence repeats over and over, like one word typed again and again in a row. Without a proofreader, these stretches get longer or shorter, and that's what's called microsatellite instability. Lynch syndrome is inherited in an autosomal dominant pattern, which means a single altered copy of the gene is enough to raise the risk. But risk isn't destiny: not everyone who carries the altered gene goes on to develop cancer.