What gene abnormality may cause mental retardation?

Down syndrome is caused by the overexpression of genes on the extra copy of chromosome 21, but many mental retardation syndromes are due to mutation in a single gene.

For instance, Fragile X syndrome, which is the most common inherited form of mental retardation, is caused by a expansion of a CGG trinucleotide repeat in the FMR1 gene on the X chromosome.

Rett syndrome is an X-linked dominant mental retardation disorder that is a common cause of severe mental retardation and autistic features in girls, and the disease is caused by mutations in the MECP2 gene.