The negatively charged Asp at the N-terminus of the peptide was chosen to aid the antigenic properties of Tyr. Lys at position 3 in the marker sequence leads to a hexapeptide sequence LysAspAspAspAspLys, which ensures a maximum value on the hydrophilicity scale according to Hopp and Woods. Such hydrophilic sequences have been shown w x to express strong antigenicity and are thus likely to adopt a highly exposed conformation in the three-dimensional folding of proteins.