CTSB and neoplasm: At present, we do not know which of the 12 endo‐lysosomal cysteine proteases could be the target for inhibition by internalized cystatin C. Concerning a possible involvement of lysosomal cysteine cathepsins in proliferation, it has been reported that the cell‐permeable cathepsin B inhibitor CA‐074Me can reduce proliferation of MCF‐10 cells and also reduce the tumor size in 3D culture [37].