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utant phenotype and the role of sodium/hydrogen ion exchange in retinal function. METHODS: Detailed histology characterized the pathological consequences of Slc9a8 mutation, and retinal function was assessed by electroretinography (ERG). A conditional allele was used to identify the cells in which NHE8 function is critical for retinal function, and mutant cells analyzed for the effect of the mutation on endosomes. RESULTS: Histology of mutant retinas reveals a separation of photoreceptors from the RPE and infiltration by macrophages. There is a small reduction in photoreceptor length and a mislocalization of visual pigments. The ERG testing reveals a deficit in rod and cone pathway function. The RPE shows abnormal morphology, and mutation of Slc9a8 in only RPE cells recapitulates the mutant phenotype. The NHE8 protein localizes to endosomes, and mutant cells have much smaller recycling endosomes. CONCLUSIONS: The NHE8 protein is required in the RPE to maintain correct regulation of endosomal volume and/or pH which is essential for the cellular integrity and subsequent function of RPE.