MOSAICISM FOR A SMALL SUPERNUMERARY RING X-CHROMOSOME IN A DYSMORPHIC, GROWTH-RETARDED MALE - MOS47,XXY 48,XXY,+R(X)/

Supernumerary ring X [r(X)] chromosomes are often found in patients wi th Turner syndrome. The phenotypic effects of the r(X) chromosome are variable, and largely depend on the presence or absence of the X inact ivation (XIST) locus. Ring(X) chromosomes in males are rare and have b een previously reported in only four cases, with 47,XY, +r(X) or mos47 ,XY, fr(X)/46,XY karyotypes. These patients all had developmental dela y and dysmorphic features. We describe a 2.5-year-old male patient wit h facial dysmorphia, growth retardation, microcephaly, global developm ental delay, and microphallus. Cytogenetic analysis from peripheral bl ood lymphocytes and fibroblasts identified mosaicism for two cell line s: mos48,XXY, +r(?X)/47,XXY. Fluorescence in situ hybridization (FISH) with an X chromosome paint showed the ring chromosome to be X chromos ome derived. This is the first case of an r(X) chromosome described in a 47,XXY patient. FISH analysis of the r(X) chromosome with an XIST p robe showed that the XIST locus was absent. Functional disomy of genes in the r(X) chromosome most likely accounts for the abnormal phenotyp e in the proband.