In some cases, it can be benecial to enable cleaving the linker to release both subunits. One version is dithiocyclopeptide, connecting transferrin and granulocyte colony-stimulating factor (G-CSF). As this linker contains as well a thrombin cleav­age site, G-CSF is released in a two-step mode in the liver. This linker can be applied for other fusion partners as well [10]. A variation in that theme is the use of a differ­ent dithiocyclopeptide including a target sequence for secretion signal proteases. This means during secretion the peptide iscleaved leaving the fusion protein just joined through the disulde bridge [11]. The furin recognition motifs is a quite use­ful sequence to release anticancer domains in proximity to malignant cells with high extracellular concentration of this protease. This furin sensitive spacer was success­fully tested with an immunotoxin [12]. Another motif that was applied in the con­text of tumor targeting is the recognition sequence of matrix metalloproteinase-2 (MMP2). In combination with a brin clot binding peptide interferon-y could thus be delivered and released to tumors [13]. The protease-rich tumor microenviron­ment can act asactivator for masked antibodies which cover their binding sites by a peptide that can be released after cleavage and thus allow binding of the antibody to its target. This prodrug concept is particularly useful for antibodies against epitopes that are not fully cancer specic and therefore would be limited dueto unwanted toxicity on normal tissue [14]. Further examples can be found with fusion proteins acting in the blood coagulation cascade that anyhow contain protease activities. The long half-life of FXIII-B subunit can be transferred to FIX by fusing both molecules together. Using a short linker sequence cleavable by activated FX (FXa), FIX can be released in the presence of trace amounts of FXa to exert its coagulating function when needed and remains in the inactive long half-life variant until then [15]. The release of a bioactive compound from a carrier or a half-life extension module such as HSA through a cleavable linker was exemplied with IFN-HSA fusions. Versions with a cleavable linker generally showed a lower concentration distribution over time but a signicantly higher bioactivity [11]. Besides extracellular proteases, intracellular ones such as furin have been exploited to split fusion proteins like immunotoxins at the linker region after their internalization to allow their endo­somal escape [12]. As unwanted protease degradation can destroy fusion proteins, it is preferable to screen for cryptic protease sites already during the design phase. Besides the elimination of protease recognition motifs, these sequences can be masked instead to make them inaccessible for the enzyme. A typical building block supporting that approach is the utilization of glycosylation sites. In the case of a peptibody containing the GLP-1 peptide, clipping at the transition to the hinge region of the Fc fusion partner was observed. Interestingly the degree of clipping varied in different cell lines. The underlying reason for that was not only the expres­sion level of host cellproteases but rather the degree of N-glcosylation in that region. In addition, highly glycosylated versions had a longer serum half-life [16].