How to reach the correct sin2 θW and αS in string theory

Effective theories with the matter content of the minimal supersymmetric Standard Model below the string scale Mstring predict a wrong value for the weak-mixing angle sin2 θW and strong coupling constant αS at the scale MZ. To resolve this problem one needs large threshold corrections. At the same time one would like to avoid introducing new intermediate scales that are small compared to Mstring. Two requests which seem to be incompatible. We show how both requirements can be satisfied in a class of (0,2) heterotic superstring compactifications with a natural choice of the vevs of the moduli fields entering the moduli dependent string threshold corrections.