Phenylalanine at position four of Aβ is important for NT4X-167 binding. Pepscan analysis using ELISA of (a, c) NT4X-167 and (b) IC16. Soluble synthetic Aβ peptides of 16 amino acids in length were probed and incubated with the antibodies. (a) NT4X-167 recognizes N-terminal truncated Aβ peptides corresponding to positions 2–4 of Aβ. Peptides starting with phenylalanine (P; position 4 of Aβ) gives the highest signal. No signal above background is seen in the absence of probed peptide and primary or secondary antibody. (b) IC16 binds to peptides with N-terminal Aβ1–4 with a preference for position Aβ2. (c) NT4X-167 signal is completely abolished by replacing phenylalanine with alanine (A) or proline (P) showing the importance of phenylalanine at position 4 for NT4X-167 binding. The signals of mutant peptides are at background signals. Abbreviation: Ab, antibody.