A U(3)_C X U(3)_L X U(3)_R gauge symmetry from intersecting D-branes
  A three-family non-supersymmetric model with U(3)^3 gauge symmetry is analyzed in the context of intersecting D-branes. This is equivalent to the `trinification' model extended by three U(1) factors which survive as global symmetries in the low energy effective model. The Standard Model fermions are accommodated in the three possible bifundamental multiplets represented by strings with endpoints attached on different brane-stacks of this particular setup. Further, a natural Quark-Lepton hierarchy is realized due to the existence of the additional abelian symmetries 
G.K. Leontaris