Popliteal pterygium syndrome: An inherited condition that is characterized by a web (pterygium) behind the knee. Facial abnormalities in popliteal pterygium syndrome are cleft palate (with or without cleft lip), pits in the lower lip, and fibrous bands in the mouth. Genital abnormalities in popliteal pterygium syndrome are underdevelopment of the labia majora, malformation of the scrotum, and failure of the testes to descend into the scrotum. Patients have an extensive web running from behind the knee down to the heel, malformed toenails, and webbed toes. The disorder is inherited in an autosomal dominant manner and is due to mutation of the interferon regulatory factor 6 gene. Also known as facio-genito-popliteal syndrome.