Disclosed is a single-chain protein comprising from amino to carboxy terminus: (a) a first binding domain derived from an immunoglobulin-like molecule or the variable regions of an immunoglobulin (b) a Fc region comprising a domain derived from an immunoglobulin CH2 domain, wherein said Fc region does not comprise a domain derived from an immunoglobulin domain (c) a linker peptide of at least 5 amino acids and (d) a second binding domain derived from an immunoglobulin-like molecule or the variable regions of an immuno globulin, wherein the first or second binding domain binds CD3.