A method of site specific labelling of an interferon molecule is provided. The method comprises the steps: a) providing a label molecule comprising a PEG moiety having an aldehyde or ketone moiety； b) providing an interferon molecule having a C terminal hydrazide moiety； and c) allowing the aldehyde or ketone moiety of the PEG moiety to react with the C terminal hydrazide of the interferon molecule to form a labelled interferon molecule, which comprises a PEG moiety attached to the C terminus of the interferon molecule via a hydrazone bond. Interferon molecules labelled using such a method are also described.
