The H+/Cl- exchange transporter CLCN3 (Borsani et al. 1995) mediates the exchange of endosomal Cl- for cytosolic H+ across late endosomal membranes, contributing to the acidification of endosomes. The activity of CLCN3 is inferred from experiments in mice (Stobrawa et al. 2001, Hara-Chikuma et al. 2005).