While we found a significantly lower level of plasma α-Klotho in the MDD patients than in the control group (714.767 ± 313.075 vs. 808.660 ± 312.629 pg/ml, P = 0.018; Supplementary Table 1), further analysis showed that the lower level was limited to the elderly first-episode MDD group compared to the age-matched elderly control group (443.871 ± 201.352 vs. 581.125 ± 172.242 pg/ml, P = 0.006), but not the recurrent elderly MDD group (586.147 ± 227.622 vs. 581.125 ± 172.242 pg/ml, P = 0.917) (Figure 1A). Here, KL is linked to major depressive disorder.