The regional localization of the gene coding for soluble acid phosphatase (ACP1) has been under debate in the two different chromosome regions, 2p23 or 2p25. Gene dosage studies in a case with a karyotype of 46,XX,dir dup(2) (p25.1----p25.3) showed that the ACP1 activity was increased to 1.4 times the mean value of normal individuals with the same ACP1 phenotype, while the level of soluble malate dehydrogenase (MDH1) was normal. These gene dosage effects indicated that the ACP1 gene locus can be mapped to 2p25.