MTHFR and homocystinuria: It is important to note that pathogenic variants in methyltetrahydrofolate reductase (MTHFR) and disorders of cobalamin metabolism can also lead to elevated Hcy levels, although with some distinct biochemical and clinical features compared to those of classic homocystinuria (Gerrard and Dawson, 2022; Sloan et al., 1993).