The present invention refers to a method for identifying hetero-multimeric ubiquitins with binding capability to a ligand. Furthermore, the invention provides DNA libraries encoding for a population of said hetero-multimeric ubiquitins as well as protein libraries obtained by expression of said DNA libraries, cells and phages containing said DNA or proteins, polynucleotides encoding for said fusion proteins and vectors comprising said polynucleotides. Further new binding proteins based on hetero-multimeric ubiquitin being able to bind specifically with high affinity to selected ligands are provided.