MTRR and homocystinuria: Autosomal recessive disorders of intracellular cobalamin metabolism (ICM), designated as cblA‐cblJ,1 can give rise, either to a deficiency of methylmalonyl‐CoA mutase (cblA, cblB, and cblD variant 2) resulting in isolated methylmalonic aciduria, or to a deficiency of methionine synthase (cblD variant 1, cblE, and cblG) resulting in isolated homocystinuria or to both, (cblC, classic cblD, cblF, and cblJ), resulting in combined methylmalonic aciduria and homocystinuria.