High-level 46XX/46XY chimerism without clinical effect in a healthy multiparous female

Chimerism, when more than one genetically distinct cell line originating in different zygotes is present in a single individual, is a rare event in humans but has been described more than 50 times in the literature. Nearly all the described cases have been detected due to discordance in the cell lines for the sex chromosomes, resulting in an ovotesticular disorder of sexual development. Recently, sex chromosome discordant chimerism detected prenatally has been reported where no phenotypic abnormality was found postnatally. We now report the finding of high-level sex chromosome discordant chimerism in a healthy pregnant 46-year-old female with two previous healthy daughters. 46XX and 46XY cell lines were detected in blood and from the buccal mucosa and chimerism was confirmed by examination of a panel of microsatellite markers. Normal fertility and the presence of female offspring are unique findings in this case and raises the possibility that chimerism with a normal phenotype may be an underappreciated outcome even in the presence of sex chromosome discordance.