This redundancy may not explain the delayed onset of symptoms observed in MLIV patients and Trpml1−/− mice, because most of the affected cell types (such as neurons and corneal epithelial cells) do not express TRPML3 [15] (AJC, NNR, TW and JGA, manuscript in preparation), and in the absence of TRPML1 they do not upregulate TRPML3 as a compensatory mechanism [58]. This evidence concerns the gene MCOLN1 and mucolipidosis type IV.