(a) Structure of affibody ZHER2 in complex with the ECD of HER2, showing D36 and D37 on the affibody in proximity to H490 of HER2. (b) Western blot analysis of in vitro cross-linking between HER2 ECD and dZHER2-36FSY or -36mFSY mutant. t: 0.5, 2, 4, and 24 h. (c) Western blot analysis of in vitro cross-linking between HER2 ECD and dZHER2-37FSY or -37mFSY mutant. t: 0.5, 2, 4, and 24 h. (d) Western blot analysis showing that increasing WT-dZHER2 decreased cross-linking between dZHER2(37mFSY) (1 μM) and HER2 ECD (1 μM).