A meta-analysis based on three different studies revealed that there was no significant difference in MCP-1 levels between BD patients and controls (Modabbernia et al., 2013); however, shortly after, another meta-analysis of five more studies concluded that MCP-1 levels are altered in BD patients but only during depressive episodes (Misiak et al., 2020). This evidence concerns the gene CCL2 and major depressive disorder.