Similarly, type 2 diabetes mellitus patients combined with major depressive disorder had a higher level of serum MCP-1 (Nguyen et al., 2021), and serum MCP-1 levels in Alzheimer’s disease patients were higher than those with mild cognitive dysfunction and healthy controls, and the serum MCP-1 levels were the highest in patients with severe Alzheimer’s disease, suggesting that higher serum MCP-1 levels were associated with more severe cognitive impairment (Lee et al., 2018). This evidence concerns the gene CCL2 and major depressive disorder.