Six genetic conditions in humans are known to lead to methionine and homocysteine elevation, that is, hypermethioninemias and hyperhomocysteinemias, as a result of deficiencies of enzymes involved in methionine metabolism (MAT, CBS, GNMT, AHCY) or affecting the mitochondrial transporter citrin and fumarylacetoacetate hydrolase (FAH; Mudd, 2011). The gene discussed is FAH; the disease is hyperhomocysteinemia.