A method of identifying a polypeptide monobody having target protein binding activity, said method comprising: providing a host cell comprising (i) a reporter gene under control of a 5′ regulatory region operable in the host cell, (ii) a first chimeric gene which encodes a first fusion polypeptide comprising a target protein, or fragment thereof, fused to a C-terminus of a DNA-binding domain which binds to the 5′ regulatory region of the reporter gene, and (iii) a second chimeric gene which encodes a second fusion polypeptide comprising a polypeptide monobody fused to a transcriptional activation domain; and detecting expression of the reporter gene, which indicates binding of the polypeptide monobody of the second fusion polypeptide to the target protein such that the transcriptional activation domain of the second fusion polypeptide is in sufficient proximity to the DNA-binding domain of the first fusion polypeptide to allow expression of the reporter gene.