Several rules are now accepted for selecting an antigenic peptide:

• 1. Although longer sequences can be used, many prefer a peptide of lengths 14-18.
• 2. Sequences at the N- or C-terminus are frequently chosen.
• 3. Hydrophilic sequences are preferred because they are readily soluble in an aqueous buffer.
• 4. Hydorphilic sequences are likely on protein surface and are accessible.
• 5. A cysteine is frequently added at the N- or C-terminus for conjugation, however, if there are two cysteines present, disulphide bonds may form inter- and intra-peptide leading to insolubility and structural alteration.
• 6. Post-translation modification site should in general be avoided.
• 7. Other suggestions include avoiding typtophan and selecting proline and tyrosine.
• 8. Peptide should be checked for the possibility of raising a cross-reactive antibody.