In addition, the level of IL-17, IL-18, and IL-21 in the GCF could be correlated with the severity of the periodontal disease; their high level in the GCF reflects an extent of destruction in periodontal tissues, though IL-21 has a particular significance, as it could be used to differentiate between periodontitis and gingivitis [250]. This evidence concerns the gene IL17A and periodontitis.