system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Which is not true about Fragile X syndrome?
assistant
The correct answer is  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.