quence which encodes an amino acid sub-sequence B wherein ??1 is an amino acid other than proline, c) the first nucleic acid fragment has a sequence which encodes an amino acid sub-sequence C wherein ??2 is an amino acid other than alanine, d) the first nucleic acid fragment has a sequence which encodes an amino acid sub-sequence D wherein ??1 is an amino acid other than lysine, e) the first nucle