ERBB2 and breast cancer: Recently, we successfully demonstrated MFLI FRET‐based optical imaging to quantify the binding of TZM to its target receptor protein HER2, which is highly expressed in HER2‐positive breast cancer cell lines.[21, 22] The MFLI data indicates that the FRET signals, upon binding of NIR‐labeled TZM FRET‐pair probes to HER2 in intact, living HER2‐positive tumor xenografts, can not only effectively delineate the tumor margins with high signal‐to‐noise ratio but also quantify the fraction of bound and internalized TZM‐HER2 complexes.[21, 22]