X1MVQGSGC1FGRX2MDRISSSSGLGC2X3 (SEQ ID NO.18), wherein X1, X2 and X3 are each independently selected from the group consisting of Lys and amino acids other than Lys, and wherein at least one of X1, X2 and X3 is Lys and at least one of X1, X2 and X3 is an amino acid other than Lys; and