Targeting radionuclide imaging of tumor-associated antigens may help to select patients who will benefit from a particular biological therapy. Affibody molecules are a novel class of small (approximately 7 kDa) phage display-selected affinity proteins, based on the B-domain scaffold of staphylococcal protein A. A large library (3 x 10(9) variants) has enabled selection of high-affinity (up to 22 pM) binders for a variety of tumor-associated antigens. The small size of Affibody molecules provides rapid tumor localization and fast clearance from nonspecific compartments. Preclinical studies have demonstrated the potential of Affibody molecules for specific and high-contrast radionuclide imaging of HER2 in vivo, and pilot clinical data using indium-111 and gallium-68 labeled anti-HER2 Affibody tracer have confirmed its utility for radionuclide imaging in cancer patients.