CTSB and breast cancer: In summary, we report on a human epidermal growth factor receptor 2 (HER2)-specific nanobody called 2Rs15d containing a His3LysHis6 segment at the C-terminus, allowing site-selective installation of the cytotoxin monomethyl auristatin E-functionalized cathepsin B-sensitive payload to provide a highly homogenous nanobody–drug conjugate, which demonstrated high potency and selectivity for HER2-positive breast cancer models.