A set of polypeptides comprising: a first P1 polypeptide comprising (i) a T1 target moiety, wherein said T1 target moiety specifically binds to an A1 antigen, and (ii) an F1 fragment of a functional domain F, in which neither said F1 fragment by itself nor said P1 polypeptide by itself is functional with respect to the function of said F domain, and a second P2 polypeptide comprising (i) a T2 target moiety, wherein said T2 target moiety specifically binds to an A2 antigen, and (ii) an F2 fragment of said functional domain F, in which neither said F2 fragment by itself nor said P2 polypeptide by itself is functional with respect to the function of said domain F, wherein said A1 antigen is different from said A2 antigen, wherein said P1 polypeptide and said P2 polypeptide do not associate with each other in the absence of a cell that has both antigens, A1 and A2, on its cell surface, in the that, upon dimerizing said fragment F1 of said polypeptide P1 with said fragment F2 of said polypeptide P2, the resulting dimer is functional with respect to the function of said domain F and wherein said fragment F1 comprises a VL domain of an antibody and said fragment F2 comprises a VH domain thereof antibody； or wherein said F1 fragment comprises a VH domain of an antibody and said F2 fragment comprises a VL domain of the same antibody.Un conjunto de polipéptidos que comprende: un primer polipéptido P1 que comprende (i) un resto diana T1, en el que dicho resto diana T1 se une específicamente a un antígeno A1, y (ii) un fragmento F1 de un dominio funcional F, en el que ni dicho fragmento F1 por sí mismo ni dicho polipéptido P1 por sí mismo es funcional con respecto a la función de dicho dominio F, y un segundo polipéptido P2 que comprende (i) un resto diana T2, en el que dicho resto diana T2 se une específicamente a un antígeno A2, y (ii) un fragmento F2 de dicho dominio funcional F, en el que ni dicho fragmento F2 por sí mismo ni dicho polipéptido P2 por sí mismo es funcional c
