Cystathionine β-synthase (CBS) deficiency, the most prevalent inborn error in the sulfur amino acid metabolism in humans [11,12,13], is biochemically characterized by severe HHcy, i.e., severely elevated levels of Hcy [14] and its metabolites, Hcy-thiolactone and N-Hcy-protein [15,16,17,18]. This evidence concerns the gene CBS and hyperinsulinemic hypoglycemia, familial, 4.