C3 and primary membranoproliferative glomerulonephritis: C3 glomerulopathy (C3G) and primary (idiopathic) immune complex membranoproliferative glomerulonephritis (IC-MPGN) are rare kidney diseases characterized by dysregulation of the complement system and progressive deposition of C3 and its breakdown products in the glomeruli, ultimately leading to kidney failure in up to 50% of patients within 10 years.