These contain the Standard Model fermions as well as additional vector fermions with GUT scale masses. If we suppose [ SU (5) × U (1) A ]/ Z 5 is a subgroup of SO (10) F, then we have the intermediate scale symmetry breaking [ SO (10) F × U (1) B ]/ Z 4 → [ SU (5) × U (1) χ ]/ Z 5 where