Abstract: Provided herein engineered polypeptides that comprise a combination of spatially-associated topological constraints, wherein at least one constraint is derived from a CD25 reference target, and methods of selecting said engineered polypeptides. Further provided are methods of using the engineered polypeptides, including as positive and/or negative selection molecules in methods of screening a library of binding molecules such as antibodies. Further provided herein are CD25 antibodies selected using these engineered polypeptides.