Supplementary Material for: The Spectrum of Mosaic Double Aneuploidy of Monosomy X and Trisomy 18: Two New Cases and Review of the Literature

Introduction: Mosaic double aneuploidy (MDA) defines the presence of two chromosomally distinct cells lines derived from the same zygote arising from early cytogenetic events. MDA for monosomy X and trisomy 18 is a rare occurrence described in only 12 individuals in the medical literature to date. Case Presentation: Here, we present two individuals with MDA of monosomy X and trisomy 18 with a Turner predominant phenotype. Congruent with previous reports, the percentage of aneuploid cells is highly variable in differing tissues, without clear genotype-phenotype associations. Molecular mechanism explored in one individual suggests two independent post-zygotic events lead to this chromosomal constitution. Conclusion: Given the rarity of this condition and the recognition that early post-zygotic chromosomal aneuploidy events are common, additional reports of individuals with MDA are needed to expand the natural history and improve counseling for families, especially in the prenatal period, as clinical manifestations of either chromosomal condition is possible.