The present invention provides methods for the expression and/or production of variable domains with a C-terminal extension that can be used for coupling of the variable domain to one or more further groups, residues or moieties. In the method of the invention a yield of at least 80% of variable domains with a cysteine containing C-terminal extension is obtained. Also variable domains are provided and polypeptides comprising one or more variable domains obtainable by the methods of the present invention, as well as compounds that comprise such variable domains and/or polypeptides coupled to one or more groups, residues or moieties.
