Although the direct effect of vitamin D on JIA synovial fibroblasts has not yet been investigated, the presence of suboptimal levels of vitamin D reduces the expression of TNF-α and IL-6, and upregulates the osteoprotegerin (OPG)/RANKL ratio in RA fibroblast-like synoviocytes, inhibiting inflammation-induced osteoclastogenesis [73]. The gene discussed is TNF; the disease is juvenile idiopathic arthritis.