A hybrid polypeptide composed of a p53 epitope peptide and a desired functional protein are produced by recombinant DNA techniques. A DNA expression vector is constructed that includes segments of DNA coding for the epitope peptide and the desired functional protein. An optional linking portion is contemplated. The linking portion of the epitope peptide is cleavable at a specific amino acid residue adjacent the functional protein by use of a sequence specific proteolytic enzyme or chemical proteolytic agent. The hybrid polypeptide expressed by the host cells transformed by the cloning vector is removed therefrom and purified by affinity chromatography techniques by use of an immobilized antibody specific to the antigenic portion of the epitope peptide. The protein is then cleaved from the isolated hybrid polypeptide with an appropriate proteolic enzyme or chemical agent, thereby releasing the mature functional protein in highly purified, highly active state.