APOE and major depressive disorder: A number of genes have been suggested to be associated with MDD, for example, the sodium‐dependent serotonin transporter and solute carrier family 6 member 4 (SLC6A4), 5‐hydroxytryptamine receptor 2A (5HT2A), apolipoprotein E (APOE), and brain‐derived neurotrophic factor (BDNF; Bosker et al., 2011; Flint & Kendler, 2014; Lopez‐Leon et al., 2008).