  A 4-dimensional heterotic string model of free fermionic construction is
presented wherein mirror symmetry breaking between observable and hidden sector
gauge groups occurs in spite of mirror symmetry between observable and hidden
sector worldsheet fermion boundary conditions. The differentiation is invoked
by an asymmetry in GSO projections necessarily resulting from the symmetry of
the free fermionic boundary conditions. In the specific examples shown, an
expected non-chiral Pati-Salam mirror universe model is transformed into a
chiral model with enhanced hidden sector gauge symmetry and reduced observable
sector gauge symmetry: [SU(4)_C x SU(2)_L x SU(2)_R]_{obs} x [SU(4)_C x SU(2)_L
x SU(2)_R]_{hid} is necessarily transformed into a chiral [SU(4)_C x
SU(2)_L]_{obs} x [SO(10) x SU(2)_R]_{hid} model because of an unavoidable
asymmetry in GSO projections.
