PCR-based genetic testing for fragile X syndrome, a triplet expansion disease. (a) The number of repeats in the fragile X locus can be determined by length analysis of PCR products. This uses PCR primers that hybridize to unique sequences bordering the region of interest, then the PCR products are analyzed by gel electrophoresis. The size of a PCR product correlates with the size of the repeated region in the gene. (b) Size of the repeated unit in the gene for eight different patients.