A prime candidate for functional studies, for instance, is a site in C5 at I1381, which not only can be explained by eculizumab binding (Fig. EV1F,G) but also is a strong determinant of lupus patient heterogeneity and CXCL10 levels (Figs. 4A and EV4A, Factor 1). Here, CXCL10 is linked to systemic lupus erythematosus.