Grand Unification of Three Light Generations

We propose an SO(18) grand-unified model that predicts three light generations, if the horizontal symmetry below the grand-unification scale is chosen to be $\mathrm{Sp}\mathrm{}\left(4\right)\mathrm{}\otimes \mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_d$. A novel feature of this model is the existence of an unbroken SU(2) gauge symmetry, under which the light generations are singlets. This raises some interesting phenomenological possibilities.