To characterize the nature of the cytochrome c1 (CYC1) processivity defect in native CIII2 assemblies we identified CYC1 peptides using mass spectrometry analysis of blue native (BN)-PAGE. Gel slices ranging from ~600-900kDa and containing CIII2 assemblies were excised from U2OS control cells, U2OS OCIAD1 knockdown cells, and U2OS OCIAD1 knockdown cells rescued with wildtype OCIAD1. Gel slices were then digested and analyzed by mass spectrometry.