SLC6A4 and periodontal disorder: Numerous studies have shown that susceptibility to periodontal diseases are affected by the polymorphisms of many genes, including interleukin-1 (IL-1) (Boukortt et al., 2015), tumor necrosis factor-α (TNF-α) (Heidari et al., 2013), transforming growth factor-beta (TGFβ) and 5-HTT (Ozer Yucel et al., 2015).