[From gene to disease; fragile X-syndrome: hereditary mental retardation due to a developmental gene].

The fragile X syndrome is the most common cause of familial intellectual disability. The identification of the 'fragile-X mental retardation' (FMR1) gene disclosed a novel genetic mechanism: an intergenerational instable cytosine-guanine-guanine (CGG) repeat leading to the absence of FMR1 protein above a threshold of 200 repeats and, subsequently, leading to familial mental retardation.