ERBB2 and breast cancer: Herein, to the best of our knowledge, we report for the first time on the quantification of the binding of HER2 to TZM to monitor HER2–TZM engagement via FLI-FRET microscopy (FLIM-FRET) as well as MFLI-FRET imaging in HER2-overexpressing AU565 breast cancer cells, both in 2D cell culture and live, intact preclinical models, respectively.