A 54-year-old Japanese woman, with a 4-year history of proteinuria and hematuria, was admitted to our hospital for renal biopsy. Laboratory examination is shown in the supplementary Table. Serum and urinary protein immunoelectrophoresis showed IgAλ monoclonal paraproteins (Supplementary Figure 1A,B). A bone marrow aspiration specimen was normocellular with 1.5% plasma cells. Histological findings of renal biopsy specimens revealed mesangial proliferative glomerulonephritis, initially diagnosed as IgA nephropathy (Figure 1A-F).