  Assuming grand unified theory (GUT) and supersymmetry, we propose a simple
model which can consistently accommodate the masses and mixings for quarks and
leptons. The grand unified group is SO(10), and $\bf 10$, $\bf 120$, and $\bar
{\bf 126}$ representations are introduced for the Higgs superfields which give
masses to the quarks and leptons. The differences of masses and mixings between
the quarks and the leptons are attributed to the Higgs boson structure. Below
the GUT energy scale, the model is the same as the minimal supersymmetric
standard model except its inclusion of dimension-5 operators for the small
neutrino masses. The renormalization group equations of the independent
parameters for the Higgs couplings with the quarks and leptons are given
explicitly to connect the two energy scales of GUT and electroweak theory.
