Familial Adenomatous Polyposis is an inherited condition where the large intestine (colon) and rectum develop many small growths called polyps, often starting in the teenage years. On their own, these polyps are not cancer, but over time, one or more of them can slowly develop further changes and turn into colon cancer. This condition is responsible for less than one in every hundred colon cancer cases overall, but it is important to recognise because, unlike most cancers, it is largely preventable once identified early.

The condition is caused by a change (variant) in a gene called APC. To understand why this matters, it helps to know what this gene normally does: it acts like a quality-control brake on cell growth in the lining of the intestine, helping cells divide at a normal, controlled pace and then stop when they should. When the APC gene isn’t working properly, this brake fails, and cells in the intestinal lining begin dividing faster than they should, eventually forming polyps.

Here’s an important detail that explains why so many polyps form rather than just one: every cell in the body of a person with this condition already carries one faulty copy of the APC gene from birth. A polyp only starts growing in a particular spot once the second, previously normal copy of the gene in that one cell also stops working, usually due to random wear and tear over time. Because the “first” faulty copy is already present everywhere in the colon, this “second hit” ends up happening independently in many different cells across the colon over the years, which is why hundreds or even thousands of separate polyps can appear, rather than just a single growth.

Familial Adenomatous Polyposis runs in families. If one parent has it, each of their children, sons and daughters equally, has a fifty percent chance of inheriting it. It doesn’t skip generations. In about one in every four to five people diagnosed, there is no family history at all; the gene change happened for the first time in that person, on its own, this situation the variant is called a de novo. This doesn’t change how the condition is managed for that individual, but it does mean their own children would still have a fifty percent chance of inheriting it going forward. The condition is estimated to affect roughly one in every ten thousand to twenty thousand people worldwide, affecting all ethnic groups equally.