Decreased numbers of optic nerve mature OLs, but not OPCs, were observed in a mouse model of sialin deficiency (Prolo et al., 2009), whereas brain region-specific transcriptomics in a mouse mucopolysaccharidosis VII model revealed changes in the expression of 53% of examined OL markers (Cahoy et al., 2008), all of which were downregulated (Parente et al., 2012). The gene discussed is SLC17A5; the disease is hyperinsulinemic hypoglycemia, familial, 4.