The invention provides methods and materials involved in displaying polypeptide sequences using viruses such as avian leukosis viruses. Specifically, the invention provides nucleic acid molecules, collections of nucleic acid molecules, polypeptides, collections of polypeptides, viruses, and collections of viruses as well as methods for making nucleic acid molecules, collections of nucleic acid molecules, polypeptides, collections of polypeptides, viruses, and collections of viruses. The invention also provides methods for obtaining displayed polypeptide sequences that interact with biological molecules and/or cells as well as methods for identifying biological molecules that interact with displayed polypeptides.