Molecules capable of specific binding to a desired target epitope are of enormous importance as both therapeutics and medical diagnostic tools. A well known example of this class of molecules is the monoclonal antibody. Antibodies can be selected that bind specifically and with high affinity to almost any structural epitope. As a result, antibodies are used routinely as research tools and as FDA approved therapeutics such that the worldwide market for therapeutic and diagnostic monoclonal antibodies is currently worth approximately $30 billion.
However, monoclonal antibodies have a number of shortcomings. For example, classical antibodies are large and complex molecules. They have a heterotetrameric structure comprising two light chains and two heavy chains connected together by both inter and intra disulphide linkages. This structural complexity precludes easy expression of antibodies in simple prokaryotic systems and requires that antibodies are produced in more elaborate (and expensive) mammalian cell systems. The large size of antibodies also limits their therapeutic effectiveness since they are often unable to efficiently penetrate certain tissue spaces. Therapeutic antibodies, because they possess an Fc region, occasionally trigger undesired effector cell function and/or clotting cascades. In addition, generating bispecific or multispecific antibodies often involves difficult and complex procedures (See e.g., Josefina et al., (1997), Nature Biotechnology, 15: 159-163; and Wu et al. (2007) Nature Biotechnology, 25: 1290-1297).
Accordingly there is a need in the art for alternative binding molecules capable of specific binding to a desired target with high affinity and specificity. A need also exists for a simple method to generate bispecific or multispecific binding molecules.