The antigenicity of a protein resides in a series of mutually overlapping surface patches known as epitopes which make contact with the combining sites of antibody molecules. Epitopes are usually localized by demonstrating the presence of antigenic cross-reactivity between a protein and some peptide fragments. Since structural features of proteins such as the accessibility, hydrophilicity and mobility of segments of the polypeptide chain have been correlated with the location of epitopes, it is possible to predict from the primary structure which linear peptides are likely to correspond to epitopes of the protein.