The present invention provides a technique for specific assembly of monomeric polypeptides to form a heteromultimer. This technique is particularly useful for generating a genetically diverse repertoire of heteromultimers such as antigen-binding units. The invention also provides both non-single-chain and single-chain antigen-binding units that are assembled by the technique described herein. The present invention also provides recombinant polynucleotides, vectors, host cells, and kits for producing the subject antigen-binding units. Further provided by the invention are methods of using the subject antigen-binding units.