The idea, approach, and proof-of-concept of the dock and lock (DNL) method, which has the potential for making a large number of bioactive molecules with multivalency and multifunctionality, are reviewed. The key to the DNL method seems to be the judicious application of a pair of distinct protein domains that are involved in the natural association between protein kinase A (PKA; cyclic AMP^dependent protein kinase) and A-kinase anchoring proteins. In essence, the dimerization and docking domain found in the regulatory subunit of PKA and the anchoring domain of an interactive A-kinase anchoring protein are each attached to a biological entity, and the resulting derivatives, when combined, readily form a stably tethered complex of a defined composition that fully retains the functions of individual constituents. Initial validation of the DNL method was provided by the successful generation of several trivalent bispecific binding proteins, each consisting of two identical Fab fragments linked site-specifically to a different Fab. The integration of genetic engineering and conjugation chemistry achieved with the DNL method may not only enable the creation of novel human therapeutics but could also provide the promise and challenge for the construction of improved recombinant products over those currently commercialized, including cytokines, vaccines, and monoclonal antibodies.The impetus for developing the dock and lock (DNL) method undoubtedly was due to the limitations of existing technologies for the production of antibody-based agents having multiple functions or binding specificities. For agents generated by recombinant engineering, such limitations could include high manufacturing cost, low expression yields, instability in serum, formation of aggregates or dissociated subunits, undefined batch composition due to the presence of multiple product forms, contaminating side-products, reduced functional activities or binding affinity/avidity attributed to steric factors or altered conformations, etc. For agents generated by various methods of chemical cross-linking, high manufacturing cost and heterogeneity of the purified product are two major concerns.We, of course, recognize that innovative fusion proteins created by recombinant technologies may be built into more complex structures to gain additional attributes that are highly desirable, yet not technically attainable, in the individual engineered construct. Well-known examples include cytokines modified with polyethylene glycol to increase serum half-lives (1), biotinylated proteins to enable immobilization into microarrays (2), and protein-DNA chimeras to quantify specific molecules to which the protein binds (3). To date, these goals are commonly achieved with varied success by judicious application of conjugation chemistries. New strategies that are based on the binding of enzyme to substrate (4) or inhibitor (5), or the high-affinity interaction between two fragments of human RNase I (6, 7), to tether two or more moieties of distinct functions i...