Fragile X is a genetic condition that results from a mutation on the FMR1 gene on the X chromosome. It is known to cause autism, mood disorders, learning disabilities, mobility challenges and other issues. As many as 100,000 Americans have the FMR1 mutation that causes fragile X disorder, while a million have the FMR1 premutation that may cause related disorders. 

The FMR1 premutation is a related condition that affects more people — as many as 1 in 150 women and more than 1 in 300 men. It can cause similar symptoms known as fragile X-associated disorders. 

Individuals may choose to be tested for fragile X if they have a diagnosis of autism, intellectual disability, tremors or fertility problems, or if they have a family history of these conditions or of fragile X disorder. The National Fragile X Foundation provides information on testing.