To extend our findings beyond the MDA-MB-231 cell model, we demonstrate that NHE1 and SWELL1 preferentially polarize at the cell front and rear, respectively, of metastatic SUM159 breast cancer cells, migrating in confinement12 and mediate isosmotic swelling and shrinkage (Supplementary Fig. 3b, c). The gene discussed is SLC9A1; the disease is breast carcinoma.