A girl with mental retardation and the facies associated with the distal 10q duplication syndrome was found to have a tandem duplication of 10q24 to 10q26. This was confirmed by gene dosage studies of glutamic oxaloacetic transaminase 1. The clinical features of this patient are compared with those of other reported cases of the distal 10q duplication syndrome with duplication-deficiency karyotypes due to familial reciprocal translocations or inversions or with tandem duplication of a more proximal region of 10q. Reports of tandem duplications in man and possible mechanisms of origin are discussed.

The hexokinase (HK) activity in human fibroblasts was close to that expected for a gene dosage effect in a mosaic cell line with about 86% trisomy 10 cells (64% greater than four control lines with normal karyotypes). There was no dosage effect for HK in the cell line that was trisomic for 10q24→qter, nor in the cell line monosomic for 10pter→p12. The data suggest an assignment of the HK1 locus (the only hexokinase in fibroblasts) to 10p11→q23 by exclusion.