A cDNA clone encoding a zinc finger protein (AREB6) was isolated from a HeLa cell expression library using a positive regulatory element (-102 to -58) of rat Na, K-ATPase α1 subunit gene (Atplal) as a probe. The clone is apparently an extended one of Nil-2-a originally isolated as a negative regulator of interleukin 2 gene [Williams, T. M. et al. (1991) Science 254, 1791-1794]. The open reading frame encodes 1, 124 amino acids. It contains 7 zinc-finger motifs arranged in two widely separated clusters. A glutamic acid-rich region is observed at the C terminus from residues 989 to 1123. Co-transfection of the AREB6 cDNA with Atplal fused to a reporter luciferase gene indicated that the AREB6 protein enhances or represses the promoter activity of the gene depending on the quantity of cDNA and on the cell type. The mRNA of AREB6 is expressed in heart and skeletal muscle, but not in liver, spleen, or pancreas. Genomic Southern analysis indicated that the gene encoding AREB6 is present as only one copy or two at most. Another cDNA clone obtained by using the same probe was identified as HEB [Hu, J. S., Olson, E. N., & Kingston, R. E. (1992) Mol. Cell. Biol. 12, 1031-1042]. Co-transfection of the cDNA enhanced or repressed the promoter activity of Atplal depending on the cell type. The binding regions of AREB6 and HEB are distinct from those of C1, C2, and C3 that were identified as binding complexes to ARE by gel retardation analysis using MDCK and B 103 cell nuclear extracts [Suzuki-Yagawa, Y., Kawakami, K., & Nagano, K. (1992) Mol. Cell. Biol. 12, 4046-4055].