C3 and systemic lupus erythematosus: To check if the fragment is still attached to or in complex with the rest of C3 in humans, we analyzed a single human SLE plasma sample (patient with lupus nephritis and a severe SLEDAI disease score of 10) fractionated by size exclusion chromatography (SEC) combined with N-terminome and proteome analysis of the corresponding fractions (Fig. EV7A,B).