The enzyme 5-lipoxygenase (5-LO) and its metabolites stimulate the formation and activity of osteoclasts in vitro [20, 21], however, 5-LO gene ablation resulted in increased bone loss in cases of apical periodontitis when root canals were infected with Gram-negative Fusobacterium nucleatum [22] or it had no impact on the enlargement of apical periodontitis, when root canals were infected with micro-organisms from oral cavity [15]. Here, ALOX5 is linked to periapical periodontitis.