Moreover, MPO was a target of drugs in five clinical trials evaluating their effectiveness in the treatment of xerostomia, head and neck cancer oral complications, and Sjögren’s Syndrome (NCT02430298, NCT00057785, NCT03953703, NCT04392622, and NCT02990468) (Table 2) [43,44,45,46,47]. The gene discussed is MPO; the disease is xerostomia.