For all these entities, pathology reveals the presence of classic non-caseating granulomas together with some peculiar clinical features (i.e., abdominal pain, diarrhea, rectal blood loss, facial nerve paralysis) [65] or laboratory tests which can help in differentiating each other (i.e., presence of proteinase-3 antineutrophil cytoplasm antibodies—PR3-ANCA, myeloperoxidase antineutrophil cytoplasm antibodies—MPO-ANCA, elevated serum angiotensin-converting enzyme (ACE) levels) [66]. The gene discussed is MPO; the disease is Granuloma.