**5.3 Affibody molecules**

Affibody molecules are 58 amino-acid, three-helix bundle affinity proteins and are derived from the β-domain of the five-domain Ig-binding region protein A from *Staphylococcus aureus (*Orlova et al., 2007). They represent highly specific binders, selected by phage display from a library generated by randomization of 13 amino acids in helix 1 and 2, which are responsible for the Fc-binding site. Recently, affibody molecules have been investigated for tumor targeting purposes both for targeted imaging and therapy. The first generated and used affibody molecule for radionuclide imaging was ZHER2 with a binding affinity of 50 nM to HER2 protein (Capalaet al., 2009). Affibody molecules represent a promising novel class of targeting molecules that can be used as relatively small, high-affinity, cancer-specific ligands and that are well suited for tumor molecular imaging and therapy, providing a possible new route for imaging of tumor-specific receptors (Orlova et al., 2006). Since these structures are derived from a *Staphylococcal* protein, the potential immunogenicity of these molecules may be a concern (Sharkey & Goldenberg, 2008).
