  We show how identification of absolutely flat directions allows the
construction of a new class of compactified string theories with reduced gauge
symmetry that may or may not be continuously connected to the original theory.
We use this technique to construct a class of 3 generation models with just the
Standard Model gauge group after compactification. We discuss the low-energy
symmetries necessary for a phenomenologically viable low-energy model and
construct an example in which these symmetries are identified with string
symmetries which remain unbroken down to the supersymmetry breaking scale.
Remarkably the same symmetry responsible for stabilising the nucleon is also
responsible for ensuring one and only one pair of Higgs doublets is kept light.
We show how the string symmetries also lead to textures in the quark and lepton
mass matrices which can explain the hierarchy of fermion masses and mixing
angles.
