HTR2A and major depressive disorder: HTR2A encodes the 5-hydroxytryptamine (also known as serotonin) receptor 2 A, which plays a crucial role in fetal brain development and adult cognitive functions (Parade et al., 2017) and whose DNA methylation in saliva has been associated with posttraumatic stress disorder and major depressive disorder symptoms in preschoolers (Parade et al., 2017).