In a further embodiment, a variant (termed HRG-NTDxGFD) is prepared wherein (1) the lysine residue found in the NTD-GFD joining sequence VKC (residues 180-182, FIG. 15) is deleted or (preferably) substituted by another residue other than R such as H, A, T or S and (2) a stop codon is introduced in the sequence RCT or RCQ (residues 220-222, FIG. 15) in place of C, or T (for HRG-??) or Q (for HRG-be