On the possible solution of the doublet-triplet problem in extended   $SU(\geq 8)$ SUSY GUTs
  The generalization of the "custodial symmetry" mechanism which leads to the natural (without fine tuning) explanation of the doublet-triplet hierarchy is suggested in the frames of extended $SU(N\geq 8)$ SUSY GUTs. It is shown also that such type of SUSY GUT can predict the value of strong gauge coupling constant $\alpha_S(M_Z)$ consistent with present experimental data. 
A. Kobakhidze