In UNC-grown CF epithelia, exposure of the apical surface to a K+-free modified Krebs solution (0K+) induced an intracellular acidification of −0.06 ± 0.01 at a rate of −0.04 ± 0.01 pH·min−1, due to the inhibition of H+ secretion by ATP12A, which accumulated in the cytosol (Figure 7D). The gene discussed is ATP12A; the disease is cystic fibrosis.