Case Report Monosomy 21 Seen in Live Born Is Unlikely to Represent True Monosomy 21: A Case Report and Review of the Literature Trent Burgess, 1 Lilian Downie, 2 Mark D. Pertile, 1 David Francis, 1 Melissa Glass, 1 Sara Nouri, 1 and Rosalynn Pszczola 2 1 Victorian Clinical Genetics Services, Murdoch Childrens Research Institute, Royal Children’s Hospital, Parkville 3052, Melbourne, Australia 2 Sunshine Hospital, Western Health, Sunshine 3020, Melbourne, Australia Correspondence should be addressed to Trent Burgess; trent.burgess@vcgs.org.au Received 27 November 2013; Accepted 22 December 2013; Published 4 February 2014 Academic Editors: P. D. Cotter, M. Fenger, and C. L´ opez Gin´ es Copyright © 2014 Trent Burgess et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We report a case of a neonate who was shown with routine chromosome analysis on peripheral blood lymphocytes to have full monosomy 21. Further investigation on ibroblast cells using conventional chromosome and FISH analysis revealed two additional mosaic cell lines; one is containing a ring chromosome 21 and the other a double ring chromosome 21. In addition, chromosome microarray analysis (CMA) on ibroblasts showed a mosaic duplication of chromosome region 21q11.2q22.13 with approximately 45% of cells showing three copies of the proximal long arm segment, consistent with the presence of a mosaic ring chromosome 21 with ring instability. he CMA also showed complete monosomy for an 8.8Mb terminal segment (21q22.13q22.3). Whilst this patient had a provisional clinical diagnosis of trisomy 21, the patient also had phenotypic features consistent with monosomy 21, such as prominent epicanthic folds, broad nasal bridge, anteverted nares, simple ears, and bilateral overlapping ith ingers, features which can also be present in individuals with Down syndrome. he patient died at 4.5 months of age. his case highlights the need for additional studies using multiple tissue types and molecular testing methodologies in patients provisionally diagnosed with monosomy 21, in particular if detected in the neonatal period. 1. Introduction Apparent full monosomy 21 has been reported in ten cases in the pre- and postnatal settings (excluding early pregnancy loss), with most cases being lethal in utero [1–14]. How- ever, many of these cases were reported when cytogenetic techniques were limited and oten only single tissues were investigated. Clinical features of monosomy 21 include severe Intrauterine Growth Retardation (IUGR), ear anomalies, clinodactyly (5th inger), seizures, and anteverted nares. Cases of monosomy 21 reported in live born, as was the provisional diagnosis in this case, are unlikely to represent true full monosomy 21. he presence of a second undetected cell line being the most likely explanation for pregnancies reaching term. he presence of a ring chromosome 21, due to their mitot- ic instability and propensity for tissue limited mosaicism, provides a plausible explanation for some of the previously reported cases of full monosomy 21. In particular, when detected in the neonatal setting. he associated phenotype in patients with mosaic ring chromosome 21 with monosomy 21 is varied. Features range from apparently normal individuals [15] through to individuals with dysmorphic features and congenital abnormalities [16]. he severity of the phenotype is likely to depend upon the prevalence and tissue distribution of the monosomy 21 cell line, the level of genomic imbalance associated with the formation of the ring chromosome, and the mitotic dynamism of the ring chromosome, also known as “Ring syndrome” [17]. 2. Case Presentation he proband, a female infant was the irst of dichorionic- diamniotic (DCDA) twin girls born prematurely at 35 + 3 weeks gestation by elective caesarean section for discordant Hindawi Publishing Corporation Case Reports in Genetics Volume 2014, Article ID 965401, 6 pages http://dx.doi.org/10.1155/2014/965401