A fibronectin type III (Fn3) polypeptide monobody, a nucleic acid molecule encoding said monobody, and a variegated nucleic acid library encoding said monobody, are provided by the invention. Also provided are methods of preparing a Fn3 polypeptide monobody, and kits to perform said methods. Further provided is a method of identifying the amino acid sequence of a polypeptide molecule capable of binding to a specific binding partner (SBP) so as to form a polypeptide:SSP complex, and a method of identifying the amino acid sequence of a polypeptide molecule capable of catalyzing a chemical reaction with a catalyzed rate constant, kcat, and an uncatalyzed rate constant, kuncat, such that the ratio of kcat/kuncat is greater than 10.