The invention relates to a modified botulinum toxin which has a naturally occurring heavy chain and a modified light chain. The invention is characterized in that the modification of the light chain is that it comprises (i) on its N terminus an extension of the chain which has the structure -(C)n-(tag)m-(X)l- in the direction from the N terminus to the C terminus, C representing a cysteine group, tag representing any tag and X representing a group of any naturally occurring amino acid, n being an integer from 1 to 50, m being 0 or 1 and l being 0 or an integer from 1 to 50, and (ii) that at least one of the cystein groups in the extension of the chain is coupled to at least one chain PEG.
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