TNFRSF11B and periodontitis: The expression levels of RANKL, RANK, and OPG mRNA were also examined, since these molecules are involved in osteoclast differentiation and it was reported that the expression of RANKL was higher and that of OPG was lower in the periodontitis group than healthy control (Vernal et al. 2006; Bostanci et al. 2007; Wara-aswapati et al. 2007).