We focused on protein purification, immunological techniques, and amino-acid-sequence determination when we last explored experimental techniques in Chapter 5. Here, we will expand our repertoire of procedures to include those used with nucleic acids—in particular, the techniques of recombinant DNA technology. We will continue our study of the estrogen receptor, a transcription factor that regulates certain genes in response to the presence of the steroid hormone estradiol. Here, we will use the purified receptor protein to isolate DNA encoding the receptor protein, to express the receptor in bacteria, to investigate the nature of its gene, and then to find out whether there are relatives of this protein in other organisms or similar proteins in the same organism.