L19 monoclonal antibody derivatives that target ED-B FN. (A) L19-IL2 immunocytokine consists of a single-chain Fv fragment fused to IL-2 molecule. (B) Single-chain Fv dimer. (C) Small immunoprotein (SIP) consists of 2 scFv fragments fused by a CH4 domain of the human IgE, which mediates its homodimerization. SIP can be further conjugated with radioisotopes to deliver radiation to the site of tumors. Professional illustration by A. Y. Chen.