A transgenic non-human animal is provided. In certain embodiments, the animal comprises a genome comprising an immunoglobulin heavy chain locus comprising: a) a transcribed gene encoding a fusion protein comprising, from N-terminus to C-terminus: i. a scaffold comprising a first binding domain; and ii. a heavy chain constant region operably linked to the scaffold; wherein the scaffold is capable of specifically binding to a target in the absence of additional polypeptides; and b) a plurality of pseudogenes that are operably linked to the transcribed gene and that donate, by gene conversion, nucleotide sequence to the part of the transcribed gene that encodes the binding domain.