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In the past three years it has been shown by recombinant DNA approaches that there are at least three different genes that code for opioid peptides. The basic structures of the three polyprotein precursor molecules from which bioactive opioid peptides are derived are remarkably similar. There are also similarities in the structure of the genes that code for these precursor molecules. Using immunological techniques, it has been shown that the levels of the opioid peptides can be regulated by altering the rates of protein processing or secretion. Recently, complementary DNA clones of the opioid peptides precursor molecules have been used as hybridization probes to determine that regulation also occurs at the level of gene expression (transcription of the opioid peptide genes).