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The only string models known to reproduce the Minimal Supersymmetric StandardModel in the low energy effective field theory are those constructed in thefree fermionic formulation. We demonstrate the existence of quasi-realisticfree fermionic heterotic-string models in which supersymmetric singlet flatdirections do not exist. This raises the possibility that supersymmetry isbroken perturbatively in such models by the one-loop Fayet-Iliopoulos term. Weshow, however, that supersymmetric flat directions that utilize VEVs of somenon-Abelian fields in the massless string spectrum do exist in the model. Weargue that hidden sector condensates lift the flat directions and breaksupersymmetry hierarchically.Comment: 20 pages. Standard Latex. References added. To appear in PR

Flat directions in left-right symmetric string derived models