Patent ID: 7244592

Claim:
A method comprising: (i) providing a plurality of initial nucleic acid cassettes comprising a) a first coding region encoding a first immunoglobulin variable domain, b) a second coding region encoding a second immunoglobulin variable domain, and c) a ribosomal binding site disposed between the first and second coding regions for translation of the second coding region in a first expression system, wherein the first and second coding regions are in the same translational orientation and can be transcribed as a single transcript; (ii) modifying each nucleic acid cassette of the plurality in a single reaction mixture so that it is functional in a second expression system, wherein the modifying comprises inserting a segment that comprises an internal ribosome entry site between the first and second coding regions and the first and second region remain physically attached during the modifying; (iii) introducing each modified nucleic acid cassette into a mammalian cell to produce a mixture of transfected cells; and (iv) expressing each modified nucleic acid cassette in the transfected cells.