a method is selects membrane - translocating peptides from a peptide display library that are capable of crossing or penetrating a lipid membrane . a plurality of nucleic acid constructs that encode displayed peptides are expressed , resulting in the formation of a plurality of nucleic acid - peptide complexes , each complex comprising at least one displayed peptide associated with the corresponding nucleic acid construct encoding the displayed peptide ; the complexes are exposed to a population of membrane - encapsulated compartments , allowing a translocating reaction to occur ; complexes that remain unassociated with the membrane are removed ; optionally complexes that are associated with the membrane are removed ; and internalized nucleic acid - peptide complexes are recovered . the membrane - encapsulated compartments may be artificial vesicles such as liposomes , or populations of one or more cell types .