Slc9a9 (Sodium/hydrogen exchanger 9) or NHE9 may act in electroneutral exchange of protons for Na(+) across membranes. Four isoforms of the Na+/H+ exchanger (NHE6-NHE9) are distributed to intracellular compartments in human cells. They are localized to Golgi and post-Golgi endocytic compartments as follows: mid- to trans-Golgi, NHE8; trans-Golgi network, NHE7; early recycling endosomes, NHE6; and late recycling endosomes, NHE9. The intracellular localization of the NHEs is established by the balance of transport in and out of the post-Golgi compartments as the dynamic membrane trafficking. Their in vivo function is to regulate the pH and monovalent cation concentration in these organelles.