International Kidney and Monoclonal Gammopathy Research Group classification of MGRS-related renal diseases. *Fibrillary glomerulonephritis (FGN) is a glomerular disorder characterized by immunoglobulin deposits composed of IgG1 and IgG4 and organized into randomly oriented fibrils of 9 to 26 nm in diameter at the ultrastructural level. These deposits coexist with abundant glomerular expression of DNAJB9, which is a sensitive and specific marker of the disease. Although the vast majority of DNAJB9-positive FGN cases feature polyclonal IgG deposits, restriction for either kappa or lambda light chain is observed by conventional immunofluorescence in around 10% of cases. FGN was initially classified as an MGRS-related renal disorder. However, a 2020 series from the Mayo Clinic in which kidney biopsies were reviewed using paraffin immunofluorescence and IgG subclass staining clearly established that DNAJB9-positive monotypic FGN is very rare and, with few exceptions, is not associated with monoclonal gammopathy.²⁸ Thus, monotypic FGN was excluded from the 2021 International Kidney and Monoclonal Gammopathy Research Group classification of MGRS-related renal disorders.³ **Examples include anti-GBM disease secondary to a monoclonal gammopathy and membranous nephropathy with monoclonal IgG deposits (with or without anti-PLA2R autoantibody activity). Cryo.GN, cryoglobulinemic glomerulonephritis; GBM, glomerular basement membrane; LCPT, light chain proximal tubulopathy. Adapted with permission from Leung et al.³