Background
Familial adenomatous polyposis (FAP), a rare autosomal dominant inherited disease, is generally considered to be related to the APC gene[1]. However, there is a special type of APC-negative but MUTYH-positive called MUTYH-associated polyposis (MAP). MAP includes both monoallelic mutations and biallelic mutations[2]. Of course, biallelic mutations lead to an increased risk of colorectal cancer. But patients with biallelic mutations are a minority, and heterozygous variants are more common. It is still unclear whether only one MUTYH mutation is of clinical importance or not. Recently, more and more research has focused on the risk of colorectal cancer caused by monoallelic mutations[3]. Previously, a number of studies showed that monoallelic MUTYH had little to do with CRC. Whether heterozygous variants increase the risk of colorectal cancer is still under debate[4]. Interestingly, what happens in the gut deserves further investigation if there is another mutation associated with colorectal polyps. In this report, we described the rare genetic variant, which were unusual genetic mutations that improve our awareness and understanding of the disease.