The other remaining fermions are vectorlike. To see this, note that with a 16 1H and a 16 ¯ ¯ − − 1 H Higgs field, we can have VEVs which breaks the GUT group down to [ SU (5) × U (1) χ ]/ Z 5. The Yukawa coupling 16 1H 16 1 10 −2 will pair up the 5 −2 and 5 ¯ ¯ 2 fermions. And we can always introduce a sterile neutrino φ which is invariant under [ SO (10) × U (1) B ]/ Z 4 and add the Yukawa coupling