RBSN and Alzheimer disease: Canonical inborn errors of cobalamin involving the lysosome, such as cblF and cblJ [14, 16, 48–50] and other lysosomal defects that impair cobalamin metabolism secondarily such as mutations in the rabenosyn-5 gene or defective lysosomal acidification in Alzheimer’s disease [12, 13], feature one or all of the classical marker metabolite trends, namely, elevated Hcy and MMA and reduced Met in cells and/or plasma.