Currently, affibody molecules binding with high affinity to several cancer-associated molecular targets, such as human epidermal growth factor receptor 2 (HER2), epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 3 (HER3), insulin-like growth factor 1 receptor (IGF-1R), platelet-derived growth factor receptor beta (PDGFRβ), and carbonic anhydrase 9 (CAIX), have been developed. The gene discussed is CA9; the disease is cancer.