CBS and hyperinsulinemic hypoglycemia, familial, 4: Cystathionine β-synthase (CBS) deficiency, a rare metabolic disease caused by mutations in the CBS gene, is characterized by severely elevated levels of homocysteine (Hcy) and its metabolites [20,21,22,23] and pathologies in the cardiovascular, skeletal (osteoporosis), and nervous systems [24,25].