Grand Unification in the SO(10) Model
  Extrapolating the coupling strengths to very high energies, one finds that they do not converge to a single coupling constant, as expected in the simplest gauge theory of Grand Unification, the SU(5) theory. We find that the coupling constants do converge, provided that two new intermediate energy scales are introduced, the energies where the group SU(4), containing the color group, and the right-isospin gauge group are spontaneously broken down. 
Alp Deniz \"Ozer and Harald Fritzsch