The present invention concerns methods and compositions for stably tethered structures of defined compositions, which may have multiple functionalities and/or binding specificities. Preferred embodiments concern hexameric stably tethered structures comprising one or more IgG antibody fragments and which may be monospecific or bispecific. The disclosed methods and compositions provide a facile and general way to obtain stably tethered structures of virtually any functionality and/or binding specificity. The stably tethered structures may be administered to subjects for diagnostic and/or therapeutic use, for example for treatment of cancer or autoimmune disease. The stably tethered structures may bind to and/or be conjugated to a variety of known effectors, such as drugs, enzymes, radionuclides, therapeutic agents and/or diagnostic agents.