In PBMCs, NR3C1 hypermethylation with reduced expression and FKBP5 hypomethylation with increased expression correlate with depression severity and suicidal ideation (Roy et al., 2017), supporting HPA-axis dysregulation as a key pathological feature and highlighting PBMC methylation as a potential biomarker (Autry and Monteggia, 2012; Monteiro et al., 2017). This evidence concerns the gene FKBP5 and major depressive disorder.