EGFR and cancer: They are extremely small (≈7 kDa), have excellent thermal and pH stability, and have been applied to cancer theranostic applications.[25,26] Nanoparticles coated with affibodies have also been explored for cancer theranostics typically via targeting human epidermal growth factor receptor 2 (HER2) and epidermal growth factor receptor (EGFR).[27–30] In particular, SPNs have also been functionalized with affibodies for theranostic purposes.