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. The gene discussed is MMACHC; the disease is vitamin B12 deficiency.