With the discovery also of many 5-HT1A iso and heteroreceptor complexes, like the 5-HT1A-5-HT7 (Renner et al., 2012), the FGFR1-5-HT1A (Borroto-Escuela et al., 2012b, 2013a, 2015c,d, 2016c) and the putative trimer complex GalR1-GalR2-5-HT1A (Millón et al., 2014, 2016), an increased understanding of the molecular basis of major depression was obtained based on the 5-HT hypothesis of depression (Carlsson et al., 1968; Figures 1, 2). This evidence concerns the gene HTR1A and major depressive disorder.