Fragile X SyndromeSelect an area in the image at right or from the list below to learn more about the causes of Fragile X syndrome.The BrainThe X ChromosomeGene FMR-1FMRP ProteinLearn MoreThe Promise of Human Genetics - Presentation (pdf)The BrainFragile X syndrome is the most common inherited cause of mental impairment. The syndrome is caused by an individual's hereditary disability to synthesize a protein called FMRP. Fragile X is an x-linked condition, which means it may be passed to children by both male and female parents.X ChromosomeFragile X is an "X-linked" condition, which means that the gene is on the X chromosome. Female parents with a premutation may pass on the gene as either a premutation or as a full mutation. Males with the premutation will pass it on to all of their daughters and none of their sons (they pass their Y chromosome on to their sons). Passed on by this chromosome, the gene responsible for fragile X syndrome is called the FMR1 (fragile X mental retardation 1) gene.Gene FMR-1The mutation is in the DNA (the chemical that makes up genes) of the X chromosome. The gene appears in three forms that are defined by the number of repeats of a pattern of DNA called CGG repeats. Individuals with less than 60 CGG repeats have a normal gene. Individuals with 60-200 CGG repeats have a premutation which means they carry an unstable mutation which can expand in future generations. Individuals with over 200 repeats have a full mutation which causes fragile X syndrome. In individuals without Fragile X syndrome, FMR-1 is able to synthesize a protein called FRMP; in those with the syndrome, the mutated FMR-1 gene is unable to do so.FMRP ProteinThe full mutation causes FMR-1 to shut down or methylate a region of the FMR-1 gene. Normally, the FMR-1 gene produces an important protein called FMRP. When the gene is turned off, the individual does not make fragile X mental retardation protein (FMRP). The lack of this specific protein causes fragile X syndrome.