ERBB2 and breast cancer: HChrR6, an optimized bacterial enzyme, converts CNOB (6-chloro-9-nitro-5-oxo-5H-benzo-(a)-phenoxazine) into the strong fluorescent and cytotoxic drug MCHB (9-p-amino-6-chloro-5H-benzo[a]phenoxazine-5-one), enabling specific targeting and significant cytotoxicity against HER2-expressing cells and markedly inhibiting the growth of orthotopic HER2 breast cancer tumors [242,243].