C3 and monoclonal gammopathy: As the putative etiopathogenetic mechanism of FGN seems to involve the interaction of DNAJB9 with misfolded IgG and C3 fragments, the most accredited hypothesis for DNAJB9-negative FGN cases reported so far involved the deposition of truncated immunoglobulins in the context of a monoclonal gammopathy (so called, heavy chain FGN) [16,26].