MTHFR and hyperinsulinemic hypoglycemia, familial, 4: Genetic deficiencies in 1-CM can cause inherited HHcy, such as the methionine synthase deficiency, folic acid synthesis deficiency, methylenetetrahydrofolate reductase (MTHFR) deficiency, and methylenetetrahydrofolate dehydrogenase, cyclohydrolase, and formyltetrahydrofolate synthetase 1 (MTHFD1) deficiency (Burda et al., 2015a; Lyon et al., 2020; Steele et al., 2020; Biesalski et al., 2022; Sudheer et al., 2022).