A collection of repeat proteins, each repeat protein comprising a repeat domain, which comprises a set of consecutive repeat modules, wherein each of the repeat modules is derived from one or more repeat units and wherein the repeat units comprise framework residues, which contribute to the folding topology of the repeat unit or contribute to an interaction with a neighboring repeat unit, and target interaction residues, which contribute to an interaction with a target substance, wherein the repeat proteins differ from other repeat proteins in the collection in at least one amino acid position of the repeat modules is described.