The peptide hormones are small proteins that regulate cellular metabolism through their specific interactions with tissues of the endocrine, nervous, and immune systems, as well as in embry onic development. During the past ten years, refinements in the techniques of recombinant DNA technology have resulted in the cloning of genes encoding approximately 50 different hormonal and regulatory peptides, including those in which the peptides themselves and the mRNAs encoding the peptides are present in only trace amounts in the tissues of origin. In addition to provid ing the coding sequences of...