Many common antidepressants including serotonin (5-HT) transporter (SERT) inhibitors such as fluoxetine and citalopram are useful for treating mood symptoms in depression (Rosenblau et al., 2012; Rizvi et al., 2013; Hieronymus et al., 2016), but they are relatively ineffective for treating motivational dysfunctions, and, in fact, it has been suggested that SERT inhibitors may exacerbate or induce these symptoms in some patients (Nutt et al., 2007; Targum and Fava, 2011; Padala et al., 2012; Stenman and Lilja, 2013; Fava et al., 2014; Rothschild et al., 2014). This evidence concerns the gene SLC6A4 and major depressive disorder.