In addition, two common MTHFR variants, namely c.665C>T p.(Ala222Val, rs1801133) and c.1286A>C p.(Glu429Ala, rs1801131), affecting amino acid residues localized within the catalytic and regulatory domains of the MTHFR enzyme, respectively, have been associated with a mild form of MTHFR deficiency and are known genetic risk factors for thrombophilia [19]. Here, MTHFR is linked to hyperinsulinemic hypoglycemia, familial, 4.