Although isolated C3 positivity without IgG, IgM, IgA, or C1q is a diagnostic criterion for C3 glomerulonephritis (C3GN), the patient’s history of MRSA endocarditis and complete recovery with antibiotic therapy support a diagnosis of C3-dominant endocarditis-associated GN. The gene discussed is C3; the disease is C3 glomerulonephritis.