For example, there is seldom a high explained variance in cG×E studies of depression that have investigated 5-HTT promoter polymorphism (Munafo et al. 2009; Risch et al. 2009; Sharpley et al. 2014; Bleys et al. 2018; Culverhouse et al. 2017). This evidence concerns the gene SLC6A4 and depressive disorder.