Fermion Mass Hierarchy without Flavour Symmetry
  We discuss a supersymmetric grand unified model which has gauge group SU(5) $\times$ SU(5) $\times$ SU(5), with matter fields transforming asymmetrically under different gauge SU(5) groups. We observe that the gauge structure of the model leads to approximate texture zero structures in fermion mass matrices and a natural hierarchy in the Yukawa couplings. The proton lifetime is estimated to be larger than $10^{34}$ years in this model. As in more conventional supersymmetric GUT models with product gauge groups, this model possesses no tensor fields with rank higher than 2, so that it might arise from a level 1 string construction. 
Chih-Lung Chou