The role of the p-glycoprotein as susceptibility factor has already been discussed in context with other psychiatric disorders and it has been hypothesized, that an impaired functionality of the p-glycoprotein may lead to an increased accumulation of particular endogenous compounds (e.g. cortisol) in the brain, which are related to psychiatric disorders such as depression [38,39]. This evidence concerns the gene ABCB1 and depressive disorder.