Editor—Constitutional mosaicism for two distinct chromosome aneuploidies is a rare cytogenetic abnormality. Usually in such cases, an autosomal aneuploidy is associated with a gonosomal aneuploidy. Little is known about the sequence of errors leading to such complex conditions. The only available studies addressing this issue concern three mosaic autosomal/gonosomal cases involving chromosome 8 (two cases) and chromosome 21 (one case), in all of which chimerism could be ruled out.1 2 A mitotic origin was inferred for both mosaic trisomy 8 cases,1 whereas the initial error in the trisomy 21 mosaic most likely occurred at meiosis.2

So far, trisomy 18 combined with monosomy X has been observed in three cases.3-5 We have recently observed a fourth patient with mosaic monosomy X/trisomy 18. We report the clinical and cytogenetic characteristics and the results of molecular analysis, which was undertaken in order to determine the origin of the aneuploid cell lines.

The female proband was the second child of healthy, unrelated parents. There was no family history of congenital anomalies or chromosome disorders. The father was 32 and the mother 34 years old at the time of her birth. She was born at term after an uneventful pregnancy and her birth weight was 3600 g. Psychomotor development was slightly delayed and she attended school up to the age of 14 years. Menarche occurred at 11 years, with regular menses up to 15 years. Thereafter, menses became progressively less frequent, until secondary amenorrhoea developed at 18 years of age.

The patient was referred for clinical and cytogenetic evaluation at the age of 23 …