MTHFR and Childhood onset: In some cases of childhood-onset PKU, classical galactosemia, arginase deficiency, SSADH, OTC deficiency, Smith–Lemli–Opitz syndrome, CTX, MTHFR, GAMT deficiency, Tango syndrome, MPS I, Mucolipidosis IV, MPS IIIA, β Mannosidosis, MPS VII, and Schindler’s syndrome, the diagnosis was not made until later in life (between 20–70 years) (n = 21).