Subsequently, we identified three key genes (CXCR6, GZMA, and KLRG1) that may be used for the diagnosis of the different periods of comorbid MDD with OA and found that the expression pattern of these three genes was different between OA and MDD, speculating that CXCR6, GZMA, and KLRG1 may present different roles during the different stages of disease development. The gene discussed is KLRG1; the disease is major depressive disorder.