Deficiency of the metabolic enzymes [e.g., betaine homocysteine methyltransferase (BHMT), cystathionine β-synthase (CBS), and cystathionine γ-lyase (CTH)] involved in these two pathways can lead to hyperhomocysteinemia (Zhang et al., 2019). The gene discussed is BHMT; the disease is hyperhomocysteinemia.