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The dopamine transporter gene (DAT1) is an important candidate gene for schizophrenia. A 40-bp VNTR (variable number of tandem repeats) polymorphism of DAT1 has been typed in 105 schizophrenic patients and 98 normal control subjects from Sichuan (China). Compared with allele frequencies for Caucasians reported in the literature, the Chinese population investigated showed a reduced frequency of the 9-copy allele and an increased frequency of the 10-copy allele. The observed frequency of genotypes was in agreement with the expected values according to Hardy-Weinberg equilibrium. No significant difference was found between patients and control subjects with regard to allele frequency, allele prevalence, and genotype counts. The results of the association study presented here are in agreement with the negative results of linkage analyses in schizophrenia pedigrees from Iceland (Kristbjarnarson et al., submitted) and from Utah (Byerley et al., 1993). Taken together, these studies suggest that variation in the dopamine transporter gene (DAT1) is unlikely to be a factor in the etiology of schizophrenia. The observed differences in allele frequencies between Chinese and Caucasian groups suggest that the human transporter gene might be useful for the construction of evolutionary trees in humans and primates as illustrated by Cavalli-Sforza's work (Mountain et al., 1992).

The D3 dopamine receptor gene is an important candidate gene for schizophrenia, since (because of its almost exclusive expression in the limbic system) it combines the dopamine receptor hypothesis with the limbic system hypothesis of schizophrenia. A BalI restriction fragment length polymorphism of the D3 dopamine receptor gene has been typed in 107 schizophrenic patients and 98 normal controls from Sichuan (China). With regard to alleles or genotypes, no significant differences were obtained between controls from Europe and China, between patients and controls, and between patient subgroups and controls. These results indicate a lack of association between schizophrenia and the D3 dopamine receptor gene in our sample. Our findings are at variance with reports of a significant excess of homozygosity at the D3 dopamine receptor gene in schizophrenic patients from Wales (United Kingdom) and Alsace (France). In conclusion, further studies will be needed with larger samples of patients from Wales and Alsace as well as with samples of different racial groups to prove or disprove the initial positive association between schizophrenia and genotypes of the D3 dopamine receptor gene.