Only about half of all patients with Turner's syndrome are monosomy 45,X on karyotyping and there are grounds for supposing that cryptic mosaicism for at least part of the Y chromosome may be present in some patients. If so this would be clinically important because of the risk to patients of gonadal neoplasms and virilisation. We have used a very sensitive method to detect Y chromosomal segments in eighteen patients with Turner's syndrome, none of whom had evidence of Y chromosomal material by cytogenetic analysis. In DNA from peripheral blood lymphocytes and/or fibroblasts we looked for specific nucleotide sequences from the sex-determining region of the Y chromosome (SRY gene) and repetitive sequences located at the centromeric region (DYZ3). By polymerase chain amplification (PCR) one patient had a definite positive signal and two patients had faintly positive signals for the SRY gene. Southern blot analysis of PCR material with a SRY-specific probe confirmed that these patients were positive for SRY and revealed another three. No patient was positive for DYZ3, suggesting that only a small portion of Y was present. These results suggest that "pure" 45,X monosomy is less frequent than previously supposed. Long-term follow-up of patients with Y sequences is needed to determine their risk for subsequent gonadal neoplasms and virilisation.
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