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This paper presents a six-year-old patient with XXXXY syndrome, whose oral findings included a cleft soft palate, hyper- or meso-taurodontism in eight primary molars and in the mandibular permanent first molars, five congenitally missing premolars, and delayed development of the permanent tooth germs. The maxillary and mandibular primary central incisors were in a cross-bite relationship. Cephalometric findings showed a short ramus of the mandible and a short maxilla in the anterio-posterior plane. The anteroposterior jaw relationship was in harmony. The cross-bite was considered to be due to the retroinclination of the maxillary primary incisors. This case emphasises the importance of regular dental care, and monitoring of facial growth and dental development in children with XXXXY syndrome.

Ullrich-Turner syndrome (UTS 45,X), Klinefelter syndrome (47,XXY), XXX, and XYY are the most common sex chromosome aneuploidies. In contrast, trisomy and tetrasomy Y are rare, and they occur predominantly in mosaic form. Dicentric Y-chromosomes are one of the most frequent Y-chromosomal rearrangements [Gersen and Keagle, 1999]. Most (91%) are found in mosaic form, usually a 45,X cell line. Two types of idic(Y) chromosomes can be discerned: those with a break in the short arm have two copies of the long arm and proximal short arm, and those with a break in the long arm have a