odes one of the substantially conserved amino acid sub-sequences designated A, B, C, D, E, F, and G in FIG. 6, the nucleic acid fragment is further characterized in that at least one of the following conditions is met,a) the nucleic acid fragment has a sequence which encodes an amino acid sub-sequence A wherein ??1 is an amino acid other than alanine, or ??2 is an amino acid other than glycine. b)