  It has been shown that the Dimopoulos-Wilczek (or missing-VEV) mechanism for
doublet-triplet splitting can be implemented in $SU(5) \times SU(5)$ models,
which requires no adjoint Higgs fields. This is an advantage from the point of
view of string theory construction. Here the stability of the gauge hierarchy
is examined in detail, and it is shown that it can be guaranteed much more
simply than in $SO(10)$. In fact a $Z_2$ symmetry ensures the stability of the
DW form of the expectation values to all orders in GUT-scale VEVs. It is also
shown that models based on $SO(10) \times SU(5)$ have the advantages of $SU(5)
\times SU(5)$ while permitting complete quark-lepton unification as in
$SO(10)$.

High Energy Physics - Phenomenology