Three peaks in the polymerase chain reaction fragile X analysis

Objective: To report and discuss the observation of three fragments on polymerase chain reaction (PCR) in routine carrier screening for fragile X. Methods: From 2005 through 2010, 34,500 women underwent prenatal screening for fragile X. PCR was carried out to amplify the repeat segment. The resulting fragments were scanned by a genetic analyser. Results: Three PCR peaks representing three different-sized fragments were found in 25 of the 34,500 women (1:1380 or 0.072%). Karyotype analysis was performed in 16 subjects. Full triple X was found in three women, while two had triple X mosaicism. Of the 16 karyotyped women, five (31%) had a finding of XXX (full or mosaic). Conclusions: Triple X (full or mosaic) is the most frequently encountered mechanism responsible for three peaks on fragile X PCR testing.