A method to determine the limits of tolerance of a protein to amino acid substitution. A cloned gene is mutated by synthesis of short segments of the gene with random base substitutions, insertion of these altered polynucleotide sequences into the gene and then transformation of cells with the mutated gene. Cells that contain a functional protein are selected and the protein (or gene) is examined to identify the successful mutation. Testing of many such short segments until the entire structural gene is examined maps the areas of tolerance and of sensitivity of the gene product to substitutions.