A further example of an FA–SMDC that does not bear a disulfide linker and is cleaved by an enzyme is a folate–camptothecin conjugate degraded by the cathepsin B enzyme.45 In addition to FA–SMDCs that are cleaved by enzymes already present in the tumour milieu, folate–enzyme conjugates have also been developed to deliver an enzyme to the folate receptor of the tumour cell prior to the administration of a prodrug that is converted to the active form by this enzyme. This evidence concerns the gene CTSB and neoplasm.