Dutzan et al. [31] showed an increase in IL-21 levels in chronic periodontitis, a positive correlation of IL-21 with PD, CAL, IL-1β, IL-6, and Th17 cytokines like IL-17 and IL-23, and a negative correlation with anti-inflammatory cytokines like IL-10 and TGF-β1 [31]. This evidence concerns the gene IL17A and periodontitis.