NPC1 and lysosomal lipid storage disorder: Such inhibition of back fusion events in U18666A-treated cells and NPC cells is expected to interfere severely with the sorting and trafficking of proteins that cycle between intraluminal and limiting membranes, as was shown for the mannose-6-phosphate receptor [24]–a situation that may well contribute to the complex pathology of NPC, and presumably of other lipidosis that are accompanied by cholesterol accumulation in late endocytic organelles [15].