In the case of IDO1, the promoter region has been well characterized and contains several cis-acting response elements that are involved in the up-regulation of the gene by cytokines, such as IFN-c, the most potent inducer of IDO activity, as well as TNF-a . Two Interferon-Stimulated Response Elements and three Gamma Activated Sequence located within the 1300-bp upstream of the ATG initiation codon appear to be critical for maximal IDO1 promoter activity, with a synergistic activation by IFN-c and TNF-a . In the present study, we identified a VNTR polymorphism in the IDO1 promoter region, consisting of a 24-bp repeat motif located 1.3-kb upstream of the ATG initiation codon. It was identified by a PCR-sequencing strategy applied to 41 DNA samples and allowed us to characterize two different alleles, named *V1 and *V2, that carry one or two repeats, respectively, the *V1 allele corresponding to the reference sequence listed in Genbank . This polymorphism appears to be common in Caucasians, the frequency of the *V1 allele being 46�C48% and that of the *V2 variant being 52�C54% in our study. No additional VNTR allele with more than two motif GSK1120212 repeats was identified. To assess the impact of the VNTR polymorphism on IDO activity, 47 males and 47 females from a cohort of 300 healthy Caucasian subjects were selected based on their genotype, and their sera were analysed to determine Trp and Kyn concentrations. We first observed a significant lower serum Trp concentration in females compared to males, as reported previously in other studies . Furthermore, females with a *V2/*V2 purchase SB203580 genotype displayed a significant and a trend toward lower serum Trp concentration compared to females with a *V1/*V2 and *V1/*V1 genotype, respectively. IDO is known to be induced by soluble hormones, such as human chorionic gonadotropin, prolactin and estrogens, supporting the hypothesis of a hormonal control of IDO expression . It can then be postulated that the VNTR polymorphism we identified has an effect on Trp metabolism under the influence of a female hormonal environment, which is partly supported by Carretti et al. who showed that circulating Trp concentration has cyclic variations throughout the menstrual cycle.