How is FXS different from other genetic disorders?
Most genetic disorders are caused by changes (mutations) in a gene that causes a change in the recipe for its protein. This causes the protein to be made incorrectly so that it does not work normally. Some genetic disorders are caused by making too much or too little of a protein due to a single change in the part of the gene that controls how much protein is made. FXS is one of a small class of genetic disorders, called trinucleotide repeat disorders, which is caused by a more complicated change in the gene. In the case of FXS, this complex change turns off the gene so no protein product is made. DNA is made up of four chemical building blocks called nucleotides: A, C, T, and G (for more details, see About genes and mutations). Trinucleotide repeat disorders have a chain of three of these nucleotides that are repeated over and over again. In most people, the number of repeats is small. If the number of repeats is large, the gene does not work properly. In FXS, the pattern of CGG is re
