Large Neutrino Mixing in Grand Unified Theories
A non-minimal, semi-realistic version of supersymmetric SU(5) grand unified theory is discussed. The solution of the doublet-triplet splitting problem leads to a better agreement between the predicted and observed values of the low-energy strong coupling constant and to a prolongation of the proton lifetime. A U(1) flavor symmetry allows to accommodate a realistic mass spectrum in the charged and in the neutral fermion sectors and protects doublet-triplet splitting and proton decay from dangerous radiative corrections or non-renormalizable operators.