The exquisite specificity of monoclonal antibodies (MAbs) has made them extremely versatile tools for scientific, diagnostic, and therapeutic purposes. However, their production is time consuming and requires the immunization of animals and the subsequent use of rodent cells. These procedures can be circumvented by the application of in vitro-selected single-chain Fv (scFv) fragments, a method increasingly used to obtain reagents principally suitable for recognizing any desired antigen (reviewed in refs. 1 –4 ).