In the MDD patients, only the elderly patients showed a significantly lower plasma level of α-Klotho (442.118 ± 207.958 vs. 570.165 ± 226.433 pg/ml, P = 0.026) in the CC genotype carriers than in the CT + TT carriers, while no association between plasma α-Klotho and genotype was found in the young MDD patients, although an association was found in young healthy controls (Table 3). Here, KL is linked to major depressive disorder.