Mosaic STS gene deletions in chorionic villus samples are often confined to the placenta, and they differ in size from STS gene deletions in patients with X-linked Ichthyosis

We report and confirm several cases of placental mosaicism for STS gene deletions. All mosaic cases with follow-up amniocentesis were confined to the placenta. Mosaic deletions likely arose from strand breaks at the common fragile site FRAXB, whereas the classical non-mosaic genotype found in patients with X-linked Ichthyosis arises from non-allelic homologous recombination during meiosis. These results support the existing hypothesis that placental mosaicism for copy number variants likely arise in common fragile sites.