NELL1 (neural epidermal growth factor-like 1) is a secreted protein that plays dual roles in bone biology and immune-mediated kidney disease. In bone, NELL1 promotes osteoblast differentiation and terminal mineralization while negatively regulating osteoblast proliferation [UniProt annotation]. However, NELL1 has gained clinical significance as a target autoantigen in membranous nephropathy (MN), an autoimmune kidney disease causing nephrotic syndrome in adults. In primary MN, circulating IgG autoantibodies against NELL1 form immune complexes that deposit along the glomerular basement membrane in a granular pattern, with NELL1 and IgG co-localizing on electron microscopy 1. NELL1-associated MN accounts for 5-10% of PLA2R- and THSD7A-negative cases 2, making it the second most common identified antigen after PLA2R 3. This subset exhibits distinctive clinical features of primary MN and is associated with malignancy, specific medications, and mercury exposure 3. Predominantly IgG1 and IgG4 anti-NELL1 antibodies can be detected via ELISA and immunofluorescence assays with high specificity (94.9%-100%) 3. Clinical recognition of NELL1-positive MN is critical, as accurate antigen identification guides treatment decisions and prognosis monitoring in this previously uncharacterized MN subset 4.