Grand Unification without Proton Decay
  It is commonly believed that grand unified theories (GUTs) predict proton decay. This is because the exchange of extra GUT gauge bosons gives rise to dimension 6 proton decay operators. We show that there exists a class of GUTs in which these operators are absent. Many string and supergravity models in the literature belong to this class. 
Andreas M\"utter, Michael Ratz, Patrick K.S. Vaudrevange
hep-ph hep-th