Which of the following is not true about Familial adenomatous polyp ?

FAP is an autosomal dominant inherited disease due to mutation of the APC gene (chromosome no. 5q21).Polyps are usually visible on sigmoidoscopy by the age of 15 years and will almost always be visible by the age of 30 yearsGardner syndrome, a variant of familial adenomatous polyposis (FAP), is an autosomal dominant disease characterized by GI polyps, multiple osteomas, and skin and soft tissue tumours. Cutaneous findings of Gardner syndrome include epidermoid cysts, desmoid tumors, and other benign tumors.Reference: page 1161 Bailey and Love&;s sho practice of surgery 26th edition