We describe a woman with Ullrich-Turner manifestations and a 45,X/46,X,+mar karyotype. Fluorescence in situ hybridization (FISH) and DNA analysis were carried out in order to determine the origin and structure of the marker. FISH showed that the marker was a Y-derived dicentric chromosome. The breakpoint at Yq11 (interval 6) was mapped using Southern blotting and polymerase chain reaction (PCR), There were no nucleotide alterations in the SRY conserved domain. Histological analysis of the gonads showed an ovarian-like stroma with no signs of testicular tissue. These findings indicate that the patient was a mosaic 45,X/46,X,idic(Yp) whose phenotypic expression, including sex determination, appeared to have had more influence from the 45,X cell line. (C) 2000 Wiley-Liss, Inc.9129598