VDR and periodontal disorder: Several studies have shown a relationship between periodontal disease and genetic factors, including polymorphisms in interleukin (IL)-1 [22], interferon-γ [23], IL-6 [24], matrix metalloproteinase (MMP)-9, tissue inhibitor of MMP-2 [25], vitamin D receptor [26], IL-1α, and IL-1β [5].