- X-linked dominant- Trinucleotide repeat CGG in FMR1 gene → hypermethylation → ↓ expression.- Most common cause of inherited intellectual disability and 2nd most common cause of genetically associated mental deficiency (after Down syndrome).

Males: 100% penetrance, females 60% penetrance

- Post-pubertal macro-orchidism- Autism- Large everted ears- Long face with a large jaw- Mitral valve prolapse- Hyperlaxity of the joints in the hand

CCG: Chin (protruding), Giant Gonads