HCFC1 and Decreased circulating vitamin B12 concentration: The complementation groups cblC, cblD and cblX are caused by mutations in the MMACHC (methylmalonic aciduria (cobalamin deficiency) cblC type, cblC), MMADHC (methylmalonic aciduria (cobalamin deficiency) cblD type, cblD) and the HCFC1 (host cell factor C1, cblX) genes.