C3 and metastatic melanoma: To investigate this, we developed and validated a competitive electro-chemiluminescence immunoassay (ECLIA) targeting a granzyme B-generated neo-epitope on type IV collagen degradation fragments (C4G) and evaluated its biomarker potential for identifying metastatic melanoma patients responding to the anti-CTLA-4 antibody ipilimumab, both alone and in combination with the fibrosis biomarker PRO-C3.