A male with trisomy 9 mosaicism and maternal uniparental disomy for chromosome 9 in the euploid cell line.

We describe a 17 year old male with a low level of trisomy 9 mosaicism. Maternal uniparental chromosome 9 disomy in the euploid cell line was shown to have arisen after postzygotic loss of the paternal chromosome 9 from the trisomic cell line by cytogenetic and molecular analysis. This is believed to be the first report of uniparental disomy for chromosome 9. In four of the 11 reported cases of mosaic trisomy 9 syndrome, including our patient, a maternally derived pericentric inversion of the heterochromatic area of chromosome 9 has been present in duplicate in the trisomic cell line. This may have implications for the counselling of patients with this common chromosomal variant.