KLB and major depressive disorder: According to the Zung Self-Rating Depression Scale (SDS), the SCAD patients were divided into a depressive group and a non-depressive group, and measured serum levels of fibroblast growth factor 21 (FGF21), β-klotho, mature brain-derived neurotrophic factor (mBDNF), and BDNF precursor (BDNF precursor, proBDNF).