The primary structure of porcine preproenkephalin B has been elucidated by cloning and sequencing cDNA1: it contains neoendorphin2,3, dynorphin4,5 and leumorphin1,6,7 (containing rimorphin as its ammo-terminus)8,9. These opioid peptides, each having a leucine-enkephalin structure10, act on the κ-receptor 7,11-14. We have now cloned a human genomic DNA segment containing the preproenkephalin B gene. The structural organization of this gene resembles those of the genes encoding the other opioid peptide precursors, that is, preproenkephalin A15 and the corticotropin-β-lipotropin precursor16-21 (ACTH-β-LPH precursor). The primary structure of human preproenkephalin B has been deduced from the gene sequence. The amino acid sequence homology observed between preproenkephalin B and preproenkephalin A, together with the similarity between their gene organizations, suggests that the two genes have been generated from a common ancestor by gene duplication.