Partial Trisomy 21 in a Patient without the Down Syndrome Phenotype. A. Umrigar1, T. J. Chen2, F. Tsien1 1) Department of Genetics, LSU Health Sciences Center, New Orleans, LA; 2) Hayward Genetics Center, Tulane School of Medicine, New Orleans, LA.

   We present a 24-year old female who was found to have trisomy 21 by routine karyotype, but did not present with the typical Down Syndrome phenotype. The patient had short stature, strabismus, and mental deficiency, but with none of the common dysmorphic features of Down syndrome. Previous results indicated that the patient had three copies of chromosome 21, one of which appeared smaller in size. Subsequent fluorescence in situ hybridization (FISH) using the Down Syndrome Critical Region (DSCR) probe showed a deletion of the 21q22.11-22.2 region on the third copy of the 21 chromosomes. Microarray comparative genomic hybridization (aCGH) revealed a deletion of approximately 13.721 Mb at 21q22. Thus, it is concluded that the patients lack of the Down Syndrome phenotype is due to this deletion. This occurrence is extremely rare, requiring both the deletion of the DSCR and a meiotic non-disjunction event of the normal chromosome. Additionally, the proband later miscarried a fetus with Potter syndrome, but we were unable to perform cytogenetic and molecular analyses of the products of conception (POC). Our case demonstrates the importance of molecular confirmation in atypical patients previously diagnosed by karyotype.