Schematic representation of FACS-based cell surface antibody binding assay of efficiently cleaved Envs with different monoclonal antibodies, (A) Env binding to bNAb versus non-NAb, (B) wild type Env versus cleavage-defective SEKS mutant binding to cleavage-dependent, trimer-selective bNAbs e.g. PGT151, PGT145, (C) wild type Env versus SEKS mutant binding to non-NAbs.