KL and major depressive disorder: As we found a significant difference in plasma α-Klotho levels as well as Klotho rs9315202 genotype between the elderly first-episode MDD and age-matched healthy control groups, to understand the impact of Klotho rs9315202 genotypes on plasma α-Klotho concentration, we divided the participants into two groups according to genotype (CC vs. CT + TT) (Table 3).