CTSB and metastatic malignant neoplasm: Cathepsin B is an enzyme of the lysosomal cysteine protease family.[46] Overexpression of this enzyme is prevalent in invasive and metastatic cancers.[47] In an effort to improve peptide linker stability of an antibody‐drug conjugate (ADC) that targets cancer cells where cathepsin B is active, Wei et al.[23] installed a cyclobutyl ring replacing a valine residue.