After prenatal detection of a de novo unbalanced translocation 46,XX,18p+, fetal fibroblasts were obtained for further studies. Chromosome banding suggested that the fetus might have trisomy of the short arm of chromosome 9. This tentative diagnosis could be confirmed by demonstration of gene dosage effects for galactose-1-phosphate uridyl transferase (GALT, EC 2.7.7.12) and nucleoside triphosphate adenylate kinase (AK3, EC 2.7.4.10) both known to be controlled by genes assigned to 9p. These findings demonstrate for the first time a gene dosage effect for AK3 in a case of 9p trisomy and show again that gene mapping information can be used to define partial aneuploidies in fetal fibroblasts.