The present invention relates to a binding molecule that specifically binds to two different epitopes of an antigen expressed on tumor cells, wherein the binding molecule comprises: (a) a first binding (poly)peptide that specifically binds to a first epitope of said antigen expressed on tumor cells, wherein said first binding (poly)peptide is a Fyn SH3-derived polypeptide; and (b) a second binding (poly)peptide that specifically binds to a second epitope of said antigen expressed on tumor cells. The present invention further relates to a nucleic acid molecule encoding the binding molecule of the invention, a vector comprising said nucleic acid molecule as well as a host cell or a non-human host transformed with said vector. The invention further relates to a method of producing a binding molecule of the invention as well as to pharmaceutical and diagnostic composition. Moreover, the present invention also relates to the binding molecule, the nucleic acid molecule, the vector or the host cell of the invention for use in the treatment of tumors.