The Abs H4B-25 and H3A-122 recognize motifs located in D3 and D4, respectively, whereas the Abs 16D3-C1 and 9F7-F11 bind to motifs within D1 of HER3. (A) Quantitative analysis of the reactivity of H4B-25, H3A-122, 16D3-C1, and 9F7-F11 toward pentadeca-peptides generated by SPOT synthesis on cellulose membranes using ImageJ. (B) Amino acid alignment of the identified Ab-binding motifs in human, monkey, and mouse HER3 and in human EGFR, HER2, and HER4. (C) The Ab-binding motifs were localized on the crystal structure of unliganded HER3 (pdb 1M6B).