[Identification of sex chromosome markers using fluorescence in situ hybridization (FISH)]

From 6 to 15% of the patients with Turner syndrome have a mosaic karyotype, i.e. a 45,X cell line and another with a small sex chromosome marker of undetermined origin which may be a ring or a centric fragment. It is important to establish whether this marker chromosome derives from a Y chromosome as this implies that the patient has a high risk of developing gonadoblastoma. The objective of the present paper was to identify the origin of small sex chromosome markers using fluorescence in situ hybridization (FISH). Eight patients were studied; seven had a Turner phenotype and one had a short stature with ambiguous genitalia. In all cases karyotype in peripheral lymphocytes showed mosaicism, with one cell line that had a sex chromosome marker, and in three cases, the mosaicism was corroborated in fibroblast cultures. Biotin labeled DNA probes with complementary centromeric alpha-satellite sequences of chromosomes X and Y were used in the FISH technique. In seven patients the chromosome marker came from the X chromosome as established with the X chromosome alpha-satellite probe. In the patient with ambiguous genitalia, the marker did derive from the Y chromosome. We conclude that the FISH technique proved to be useful to establish the origin of sex chromosome markers in our laboratory.