Disclosed is a method that includes: (i) providing a plurality of initial
nucleic acid cassettes that include: 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 polypeptide in a first expression system, wherein the first
and second coding regions are in the same translational orientation; (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 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.