Protein synthesis inhibitor resistance genes (typified by a cycloheximide resistance gene) are capable of imparting resistance to a protein synthesis inhibitor (typified by cycloheximide) to animal cells sensitive to the inhibitor. The genes have a sequence mutated by substitution in a gene encoding a ribosome-constituting protein derived from an animal. The genes may be placed in recombinant vectors, including expression vectors containing the gene together with a foreign protein structural gene.