A New Doublet-Triplet Splitting Mechanism for Supersymmetric SO(10) and   Implications for Fermion Masses
  We present a new mechanism for doublet-triplet splitting in supersymmetric SO(10) models using a missing vev pattern which is different from the one used in the currently popular Dimopoulos-Wilczek method. In our method, the doublets in a {$\bf 16,\bar{16}$} pair are the ones split from the rest of the multiplet and are then mixed with the doublets from one or two {\bf 10}'s giving rise to the doublets {$\bf H_u$} and {$\bf H_d$} of the standard model. This approach provides a natural way to understand why top quark is so much heavier than the bottom quark. It also enables us to generate both hierarchical and nonhierarchical pattern for neutrino masses, the latter being of interest if neutrino is the hot component of the dark matter of the universe. We construct a simple, realistic model based on this idea. The model uses only simple representations, has no unwanted flat directions and maintains coupling constant unification as a prediction. 
Z. Chacko and Rabindra N. Mohapatra