A 6.5-kilobase murine genomic DNA fragment isolated by Levenson et al. (Levenson, R., Racaniello, V., Albritton, L., and Housman, D. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 1489-1493) (called the ouabain resistance gene) has been shown to produce ouabain resistance in primate cells. Preliminary sequence information has revealed no homology with the coding sequence of the Na,K-ATPase. We have introduced this murine sequence into monkey and murine cells in an attempt to characterize its mechanism of action. In our experiments, transfection of this DNA fragment is associated with the low frequency (1 in 8 x 10(5) cells) appearance of ouabain-resistant clones of CV1, COS, and NIH 3T3 cells, an event not seen in control transfections. Characterization of a new clone of ouabain-resistant CV1 cells (called OR8 cells) revealed a 5-fold increase in the IC50 for ouabain inhibition of rubidium uptake and a 10-fold increase in cell survival on ouabain. Although the murine sequence was detectable in Southern blots of ouabain-resistant cells soon after transfection, this exogenous DNA was rapidly lost despite continued exposure to ouabain. Furthermore, we were unable to detect message expression by this genomic sequence in any of the three cell types tested. Instead, we found that all three ouabain-resistant cell lines exhibited point mutations in a domain of the alpha-subunit that has been implicated in ouabain sensitivity (H1-H2). One of these mutations (Asp121-Asn121 in OR8 cells) has been previously reported to cause ouabain resistance (Price, E.M., Rice, D.A., and Lingrel, J.B. (1989) J. Biol. Chem. 264, 21902-21906). Other novel mutations in the H2 transmembrane domain were also detected. We postulate that the "ouabain resistance gene" is important in the early selection process on ouabain but that the permanent ouabain-resistant phenotype is due to a stable mutation in one allele of the alpha-subunit of the Na,K-ATPase.