  The experimental data of the past decade suggests that the quantum gravity
vacuum should possess two key ingredients. The existence of three generations
and their embedding in SO(10) representations. The Z2XZ2 orbifold of the
heterotic string provides examples of vacua that accommodate these properties.
The utilization of string dualities to explore these models is discussed.
Classification of the Z2XZ2 orbifold with geometric shifts on complex manifolds
demonstrates that three generations are not obtained solely with symmetric
shifts on complex tori, but necessarily utilize an asymmetric shift or a
nonperturbative breaking of the GUT gauge group. The issue of mass and mixing
spectrum in the neutrino versus the quark sector is discussed.
