Complete sequencing of APC gene belonging to the chromosome 5 from the circulatory lymphocytes is now the consecrated genetic test for the FAP syndrome.Faced with a patient with colonic polyposis, but without APC gene mutation, we must search for the MYH gene mutation belonging to chromosome 1 from the circulatory lymphocytes. The gene discussed is MUTYH; the disease is Familial adenomatous polyposis.