Given that IL-17-expressing CD4+ T helper 17 (Th17) cells and FOXP3+CD4+ regulatory T (Treg) cells play critical roles in the pathogenesis of both periodontitis and arthritis,63,73 we examined whether the R878H mutation affects the differentiation of naive T cells to Th17 or Tregs. The gene discussed is IL17A; the disease is arthritic joint disease.