To determine if the biphasic modulation of NHE1 by NO is a more general property of NHE1-expressing cells, measurements were performed on cultured neonatal rat ventricular myocytes and on a human breast cancer cell line MDA-MB-468, both of which produce NHE1 fluxes that are comparable in magnitude to those in adult ventricular myocytes. The gene discussed is SLC9A1; the disease is breast cancer.