Flavor Physics in SO(10) GUTs with Suppressed Proton decay Due to Gauged   Discrete Symmetry
  Generic SO(10) GUT models suffer from the problem that Planck scale induced non-renormalizable proton decay operators require extreme suppression of their couplings to be compatible with present experimental upper limits. One way to resolve this problem is to supplement SO(10) by simple gauged discrete symmetries which can also simultaneously suppress the renormalizable R-parity violating ones when they occur and make the theory "more natural". Here we discuss the phenomenological viability of such models. We first show that for both classes of models, e.g the ones that use ${\bf 16}_H$ or ${\bf 126}_H$ to break B-L symmetry, the minimal Higgs content which is sufficient for proton decay suppression is inadequate for explaining fermion masses despite the presence of all apparently needed couplings. We then present an extended ${\bf 16}_H$ model, with three {\bf 10} and three {\bf 45}-Higgs, where is free of this problem. We propose this as a realistic and "natural" model for fermion unification and discuss the phenomenology of this model e.g. its predictions for neutrino mixings and lepton flavor violation. 
R. N. Mohapatra and A. T. Azatov