Abstract: A method is provided for performing saturation mutagenesis on a target gene by exploiting the immunoglobulin hypermutation system. A target gene is cloned into an expression vector containing immunoglobulin enhancer fragments that effect hypermutation, and this construct is then transfected into an immunoglobulin mutator cell, typically of pre-B lymphocyte lineage. The target gene is permitted to hypermutate at a rate approaching that of 10.sup.-4 /bp/generation as the cells are cultured to a desired density. The variant polypeptides encoded by the hypermutated target gene can then be selected.