The physiologic expression of the human multidrug resistance MDRI gene product P-glycoprotein is controlled in a tissue-and cell-specific manner, but the regulatory mechanisms have not been characterized in great detail. Studies by Kohno e t al. [( 1990) J Biol Chem 265: 19690-196961 suggested that a tissue-specific enhancer element located approximately I0 kb upstream from the major MDRI transcription start site may act t o increase the levels of transcription in cultured adrenal and kidney cells. Using this putative "MDR enhancer" as a probe, we isolated a 14 kb DNA fragment from a genomic D N A library prepared from human fetal liver. The restriction map and partial nucleotide sequence of this DNA fragment were consistent with the previously described data obtained for a similar piece of genomic D N A derived from human placenta by Kohno et al. (ibid.). Pulsed-field gel electrophoresis of large genomic DNA fragments, however, showed that the DNA sequences, including the putative "MDR enhancer," were not linked t o the MDRI gene. Fluorescence in situ hybridization analysis revealed that this enhancer-like element is located on chromosome 20 at band q 13. I and is, therefore, distinct from the MDR locus on chromosome 7, band q2 I. I. Thus, this putative regulatory element does not modulate the tissue specificity of expression of the MDRI gene in vivo, but may play a role in the regulation of expression of another, so far unknown gene. Genes Chromosom Cancer 10:267-274 (1994).