Supersymmetric SO(10) Simplified

In the context of supersymmetric $SO(10)$ grand unified models, it is shown that the gauge symmetry breaking as well as a natural doublet--triplet splitting can be achieved with a minimal Higgs system consisting of a single adjoint and a pair of vector and spinor multiplets. Such a Higgs spectrum has been shown to arise in the free fermionic formulation of superstrings. Since the symmetry breaking mechanism relies on non--renormalizable operators, some of the Higgs particles of the model turn out to have masses somewhat below the GUT scale. As a consequence, the unification scale is raised to about $2 \times 10^{17}~GeV$ and sin$^2\theta_W$ is predicted to be slightly larger than the minimal SUSY--$SU(5)$ value. Including threshold uncertainties, which turn out to be surprisingly small in the model, we show that sin$^2\theta_W$ prediction is consistent with experiments.