During periodontitis, host cells produce excessive amounts of reactive oxygen species (ROS), such as those produced by myeloperoxidase (MPO), 8-hydroxydeoxyguanosine (8-OHdG) and inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin (IL)-6, in response to bacterial infiltration. This evidence concerns the gene MPO and periodontitis.