  We analyse the phenomenology of an exemplary exophobic Pati-Salam heterotic
string vacuum, in which no exotic fractionally charged states exist in the
massless string spectrum. Our model also contains the Higgs representations
that are needed to break the gauge symmetry to that of the Standard Model and
to generate fermion masses at the electroweak scale. We show that the
requirement of a leading mass term for the heavy generation, which is not
degenerate with the mass terms of the lighter generations, places an additional
strong constraint on the viability of the models. In many models a top quark
Yukawa may not exist at all, whereas in others two or more generations may
obtain a mass term at leading order. In our exemplary model a mass term at
leading order exist only for one family. Additionally, we demonstrate the
existence of supersymmetric F- and D-flat directions that give heavy mass to
all the colour triplets beyond those of the Standard Model and leave one pair
of electroweak Higgs doublets light. Hence, below the Pati-Salam breaking
scale, the matter states in our model that are charged under the observable
gauge symmetries, consist solely of those of the Minimal Supersymmetric
Standard Model.
