Three peaks in the polymerase chain reaction fragile X analysis

To report and discuss the observation of three fragments on polymerase chain reaction (PCR) in routine carrier screening for fragile X.

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.

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).

Triple X (full or mosaic) is the most frequently encountered mechanism responsible for three peaks on fragile X PCR testing.