One possibility is that this SNP manifests its effects only in presence of a stronger insult such as the viral infection; in other words, this may be a case of a gene (NOX4 rs3017887)-environment (HCV) interaction: the SNP and HCV jointly influence the risk of developing MS, as already described for other conditions, for example, two SNPs in the CYP1A1 gene that are associated with increased risk of HCC only in cigarette smoking subjects [18]. The gene discussed is NOX4; the disease is viral infectious disease.