True about Fragile X syndrome:
Fragile X syndrome is the prototype of the disease in which the mutation is characterized by a long repeating sequences of three nucleotides -
"Fragile sites are a region of the chromosome that show a tendency for separation, breakage or attenuation under particular growth conditions. One fragile site (of a total of 120 in the human genome) that has clinical significance is that on the distal long arm of chromosome Xql 7.3 associated with fragile syndrome"-
Nelson 18th/515
Fragile X Syndrome - Second most common genetic causes of mental retardation, after Down syndrome
The only distinctive feature that can be detected in at least 90% of postpubertal males with fragile -X Syndrome is macroorchidism
 Long face with a large mandible & large everted ears
 Hyperextensible joints, a high arched palate & mitral valve prolapsed is noted in some patients As with all X-linked disease fragile X syndrome affects males
Chromosomal breaking
Mitochondrial mutation
Triplet nucleotide repeat sequence
Centrochrome absent