Trisomy 10qter confirmed by in situ hybridisation.

We report a boy with multiple congenital anomalies compatible with trisomy for the distal region of the long arm of chromosome 10 and a male karyotype with one 18p+. In situ hybridisation with a cDNA for ornithine aminotransferase (OAT), whose locus maps to 10q26, confirmed the clinical suspicion of distal trisomy 10q. Subterminal localisation of the labelling signals on chromosome 10 and on the der(18) indicated the localisation of the OAT locus in the proximal part of 10q26. Two clusters of labelling signals were also found on the pericentromeric and proximal portion of the X chromosome short arm, thus confirming the presence in this region of two non-adjacent OAT pseudogenes. The phenotypic similarities of this patient to previously reported cases provide further support for the delineation of trisomy 10qter as a specific, clinically recognisable syndrome.