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. Here, MTR is linked to homocystinuria.