ß 2007 Wiley-Liss, Inc. American Journal of Medical Genetics Part A 143A:727–733 (2007) Clinical Report Pure and Complete Trisomy 18p Due to a Supernumerary Marker Chromosome Associated With Moderate Mental Retardation P. Mabboux, 1 * S. Brisset, 1 A. Aboura, 1 D. Pineau, 1 V. Koubi, 3 S. Joannidis, 4 P. Labrune, 2 and G. Tachdjian 1 1 Service d’Histologie Embryologie Cytoge ´ne ´tique, INSERM U782, Ho ˆpital Antoine Be ´cle `re, APHP; Universite ´ Paris Sud, Clamart, France 2 Service de Pe ´diatrie et Consultation de Ge ´ne ´tique, Ho ˆpital Antoine Be ´cle `re, APHP, Clamart, France 3 Laboratoire Pasteur Cerba, De ´partement de ge ´ne ´tique, Cergy Pontoise, France 4 Service de Me ´decine Interne, Ho ˆpital Ge ´ne ´ral d’Etampes, Etampes, France Received 28 June 2006; Accepted 25 November 2006 Trisomy for the short arm of chromosome 18 or trisomy 18p, is rarely described. We report on a 13-year-old boy with minor facial anomalies, mental retardation, bilateral cryp- torchidism associated with a de novo supernumerary marker chromosome (SMC). Using fluorescence in situ hybridization and comparative genomic hybridization analyses, this SMC corresponded to the p arm of chromosome 18 associated with a centromere of either chromosome 13 or 21 and nucleolus organizing regions (NORs). We report here the first case of a pure and complete trisomy 18p due to a SMC. This report and review of literature confirm that the main phenotypic anomaly associated with trisomy 18p is moderate mental retardation. ß 2007 Wiley-Liss, Inc. Key words: chromosome 18; trisomy 18p; supernumerary marker chromosome; mental retardation How to cite this article: Mabboux P, Brisset S, Aboura A, Pineau D, Koubi V, Joannidis S, Labrune P, Tachdjian G. 2007. Pure and complete trisomy 18p due to a supernumerary marker chromosome associated with moderate mental retardation. Am J Med Genet Part A 143A:727 –733. INTRODUCTION Trisomy for the short arm of chromosome 18 or trisomy 18p has rarely been reported. The first case was published by Jacobsen and Mikkelsen [1968]. Since this publication, trisomy 18p has been reported in 20 other cases [Taylor et al., 1975; Rosano et al., 1977; Gardner et al., 1978; Hernandez et al., 1979; Meinecke and Koske-Westphal, 1981; San Martin et al., 1981; Habedank and Trost-Brinkhues, 1983; Johansson et al., 1988; Takeda et al., 1989; Wolff et al., 1991; Moog et al., 1994; Li et al., 1998; Oner et al., 2000]. Trisomy 18p was due to different chromosomal mechanisms such as unbalanced segregants from parental translocation, duplications, supernumerary marker chromosomes (SMC) or complex chromosomal anomalies. Due to these different mechanisms and the size of the chromo- somal segment of the partial trisomy 18p, trisomy 18p does not constitute a specific phenotypic entity. Phenotypic expression varies markedly and is characterized by mental retardation, foot or hand anomalies and craniofacial anomalies. We report on a 13-year-old boy with minor facial anomalies, mental retardation, bilateral cryptorchid- ism associated with a pure and complete trisomy 18p due to a SMC. CLINICAL REPORT The propositus, a 13-year-old boy, was referred because of mental retardation. He was the first child of healthy unrelated parents and was born by vaginal delivery after a full-term uneventful pregnancy. There was no familial history of either mental retardation or congenital malformations. Birth weight and height were 3,700 g and 52 cm, respectively. Apgar score was 10 at one and five minutes. The neonatal period was uneventful. *Correspondence to: P. Mabboux, Service d’Histologie Embryologie Cytoge ´ne ´tique, INSERM U782, Ho ˆpital Antoine Be ´cle `re, APHP, 157, rue de la porte de Trivaux, BP 405, 92141 Clamart Cedex, France. E-mail: philippe.mabboux@abc.aphp.fr DOI 10.1002/ajmg.a.31633