Mosaicism for trisomy of chromosome 13, the existence of normal and trisomic cells in the same individual, is a very rare syndrome in liveborns presenting with variable clinical severity. However, recent studies in IVF reveal that embryonic mosaicism is not as rare, and may also have more clinical implications than previously thought. In this study, we utilized SNP microarray to investigate the origin of mosaic trisomy 13 in three patients with varying phenotypes and cytogenetic findings, and were able to determine the timing of the error (nondisjunction), parental origin, propose mechanistic models, and calculate the percentage of mosaicism for each case. Such information may lead to a better understanding of mechanisms underlying mosaic trisomies and monosomies, the observed clinical variability, and better prediction of recurrent risk. (By Dr. Denise Batista, http://jmg.bmj.com/content/early/2010/11/19/jmg.2010.083931.abstract?papetoc )