Previous studies have shown that human ornithine decarboxylase (ODC)-encoding sequences map to two chromosome regions: 2pter-p23 and 7cen-qter. In the present work we have cloned the expressed human ODC gene from a genomic library of myeloma cells that overproduce ODC protein due to selective gene amplification and determined its entire nucleotide sequence. The gene comprises 12 exons and 11 introns and spans about 8 kb of chromosome 2 DNA. The organization of the human gene is very similar to that of the mouse and rat, with the major difference being the presence of longer intronic sequences in the human gene. Some of these differences can be accounted for by the insertion of four Alu sequences in the human gene. Several potential regulatory elements are present in the promoter region and in 5'-proximal introns, including a TATA box; GC boses; AP-1-, AP-2- and NF-1-binding sites; and a cAMP-responsive element. The 5'-untranslated sequence of ODC mRNA is extremely GC-rich, and computer predictions suggest a very stable secondary structure for this region, with an overall free energy of formation of -225.4 kcal/mol. In addition to the active ODC gene on chromosome 2, ODC gene-related sequences were isolated from human chromosome 7-specific libraries and shown to represent a processed ODC pseudogene.