Secondly, there is a possibility that association between HTR3A c.-42T allele and severe dyspeptic symptoms is due to the effect of some other polymorphism, even in another gene, which is in linkage disequilibrium with HTR3A c.-42C > T. Indeed, the serotonin receptor subunit gene HTR3B maps in close proximity to HTR3A and the possibility has been raised that the HTR3A c.-42C > T polymorphism is not the susceptibility variant but a common variant in HTR3B, found to be associated with depression in Japanese [54]. Here, HTR3B is linked to major depressive disorder.