Twenty-five years ago, Georges Köhler and César Milstein invented a means of cloning individual antibodies, thus opening up the way for tremendous advances in the fields of cell biology and clinical diagnostics (1). However, in spite of their early promise, monoclonal antibodies (MAbs) were largely unsuccessful as therapeutic reagents resulting from insufficient activation of human effector functions and immune reactions against proteins of murine origin. These problems have recently been overcome to a large extent using genetic-engineering techniques to produce chimeric mouse/human and completely human antibodies. Such an approach is particularly suitable because of the domain structure of the antibody molecule (2), where functional domains carrying antigen-binding activities (Fabs or Fvs) or effector functions (Fc) can be exchanged between antibodies (see Fig. 1).