  A D-parity violated SUSY SU(4) x SU(2) _ L x SU(2)_R gauge model with the
Higgs sector 2{(4,1,2)+(4-bar,1, 2)}+(1,2,2)+some(1,1,1)'s is shown to have the
following advantages: (i) It is the simplest and almost unique solution that
satisfies M_X = M_{string} ~0.6 x 10^{18} GeV and M_{INT} ~ 5 x10^{11} GeV in
superstring derived SUSY SO(10) GUTs. (ii) The proton is stable enough by the
automatic "doublet-triplet splitting" closely connected with the D-parity
violation. (iii) The minimization of SUSY one-loop effective potential in a toy
model suggests that the SO(10) gauge theory tends to break dynamically down to
the SU(4) x SU(2)_ L x SU(2)_R model.
