Abstract: The present invention relates to a method for preparing a labelled polypeptide, the method comprising: a. providing a polypeptide comprising: i. a sortase acceptor site or a sortase donor site; ii. a non-cytotoxic protease or a proteolytically inactive mutant thereof; iii. a Targeting Moiety (TM) that is capable of binding to a Binding Site on a target cell; and iv. a translocation domain; b. incubating the polypeptide with: a sortase; and a labelled substrate comprising a sortase donor site or a sortase acceptor site, respectively, and a conjugated detectable label; wherein the sortase catalyses: conjugation between an amino acid of the sortase acceptor site of the polypeptide and an amino acid of the sortase donor site of the labelled substrate; or conjugation between an amino acid of the sortase acceptor site of the labelled substrate and an amino acid of the sortase donor site of the polypeptide; thereby labelling the polypeptide; and c. obtaining the labelled polypeptide.