A polypeptide is provided, which has a binding affinity for HER2 and which
is related to a domain of staphylococcal protein A (SPA) in that the
sequence of the polypeptide corresponds to the sequence of the SPA domain
having from 1 to about 20 substitution mutations. Nucleic acid encoding
the polypeptide, as well as expression vector and host cell for
expressing the nucleic acid, are also provided. Also provided is the use
of such a polypeptide as a medicament, and as a targeting agent for
directing substances conjugated thereto to cells overexpressing HER2.
Methods, and kits for performing the methods, are also provided, which
methods and kits rely on the binding of the polypeptide to HER2.