The 70-kDa peroxisomal membrane protein (PMP70) is a member of a family of half-ATP-binding cassette (ABC) transporter proteins located in the human peroxisomal membrane. Other members include the PMP70-related peroxisomal membrane protein, the adrenoleukodystrophy protein (ALDP), and the adrenoleukodystrophy-related protein. The functions of ABC transporters in the peroxisomal membrane are poorly understood. Evidence from yeast and human mutants suggests that they are involved in the peroxisomal import of fatty acids and/or fatty acyl-CoAs into the organelle. We report the cloning and characterization of the human PMP70 structural gene (gene symbol: PXMP1) localized on human chromosome 1p21-p22. PXMP1 is approximately 65 kb in length, contains 23 exons, and is quite different in structure from the gene (ALD) that encodes the related protein, ALDP. We also analyzed the 5' flanking region of the human PXMP1 gene and the corresponding region of murine Pxmp-1. Both promoters have features of housekeeping genes, including a high GC content and multiple consensus Sp1 binding sequences. In more than 3 kb of Pxmp-1 5' flanking sequence we did not identify a consensus peroxisomal proliferator responsive element.