MONDO_0018964 (homocystinuria without methylmalonic aciduria) is an inborn error of vitamin B12 (cobalamin) metabolism characterized by megaloblastic anemia, encephalopathy and, sometimes, developmental delay, and associated with homocystinuria and hyperhomocysteinemia. There are three types of homocystinuria without methylmalonic aciduria; cblE, cblG and cblD-variant 1 (cblDv1). Also known as: functional methionine synthase deficiency, methylcobalamin deficiency.