The heritable disorder fragile X syndrome,a major cause of mental retardation,is caused by

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Fragile X syndrome is caused by a mutation in the FMR1 gene, which contains a repeated sequence of three nucleotides (CGG). In individuals with fragile X syndrome, the CGG sequence is repeated many more times than normal, leading to the gene being "turned off" and not producing enough of the FMRP protein. This protein is important for proper brain development and function, and its deficiency leads to the various symptoms of fragile X syndrome, including intellectual disability.

The Heritable Disorder Fragile X Syndrome,a Major Cause of Mental