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.