For example, a patient with high serum IgG4, MPO-ANCA positivity, chest nodules, and elevated creatinine was diagnosed with IgG4 related kidney disease (IgG4-RKD) and MPA overlap syndrome based on renal biopsy findings (101).In some GPA cases, elevated serum IgG4 levels, fibrosis, vascular occlusion, and IgG4-positive plasma cell infiltration mimic IgG4-RD’s clinical histopathological features (102–104). The gene discussed is MPO; the disease is granulomatosis with polyangiitis.