HCFC1 and methylmalonic acidemia with homocystinuria, type cblX: Mutations in Host Cell Factor C1 (HCFC1) transcription factor cause Methylmalonic aciduria and homocystinuria (MIM #309541, also known as Mental retardation, X-linked 3), an X-linked recessive metabolic disorder characterized since early infancy by severe delayed psychomotor development.