MCOLN1 and retinal degeneration: Given the complex clinical presentation of MLIV, including retinal degeneration, optic nerve pathology, and malfunctioning of parietal cells in the stomach leading to achlorhydria, a therapeutic approach involving systemic administration of CPP16-MCOLN1 that simultaneously targets the CNS, the eye, and peripheral organs may help to elicit maximal benefits in MLIV in the clinical setting.