Which is not true about Fragile X syndrome?

Fragile - X syndrome Fragile - X syndrome is the prototype of diseases in which the mutation is characterized by a long repeating sequence of three nucleotides. In fragile X syndrome, trinucleotide repeat mutation involves CGG on the non-coding region. Clinical features of fragile - X syndrome Mental retardation   Long face with large mandible Hyperextensible joint  Mitral valve prolapse Large everted ears Large testis (macroorchidism)  High arched palate        Fragile X syndrome is the second most common cause of mental retardation, after Downs syndrome.