MTHFS and hyperinsulinemic hypoglycemia, familial, 4: Here, we present clinical findings of three Chinese patients with MTHFS deficiency, identify four novel variants c.504del (p.Y169Tfs*17), c.158C>T (p.S53F), c.117+1del and c.182A>G (p.E61G) in the MTHFS gene, and review the literature on the clinical features and therapy responses of this disease.