Fragile X syndrome is one of the most common genetic causes of intellectual disability. An X-linked trait, it is more commonly seen in males. Females may carry the fragile X mental retardation 1 (FMR1) gene without exhibiting any of its characteristics; however, they can also be as severely affected as males. This syndrome takes its name from the small area on the long arm of the X chromosome that looks like a break in the arm (although it actually is not). The cells need to be grown in a special medium to reveal this pattern; a regular karyotype will miss it. Even with the special medium, not all cells show the characteristic. In female carriers of this trait, the syndrome becomes harder to detect as the woman ages. Accurate detection of fragile X syndrome at a molecular level is now available.

Rare conditions such as excess chromosome breakage (Fanconi anemia) or abnormal centromeres (Roberts syndrome) merit special analytic processes and procedures. Chromosome and molecular analyses are done using a venous blood sample (5 mL with ethylenediaminetetraacetic acid tube) to identify the FMR1 gene and possible carrier status.