Disclosed is a PEG-linked polypeptide comprising an antibody single variable domain, wherein the variable domain comprises a framework region and has an antigen binding site, and said variable domain binds antigen as a single antibody variable domain in the polypeptide, and further wherein said PEG is present as one or more molecules linked to the polypeptide at a cysteine or lysine residue in the framework regions of said antibody single variable domain.
