Fermion Masses in Superstring Theory
  We give a model-independent discussion of fermion masses in four-dimensional heterotic superstring theories. We discuss the tree level contributions and quantum corrections, including one-loop threshold effects and masses generated as a result of non-perturbative supersymmetry breaking. We also point out that superstring models give rise to a generic $\mu$-term in the effective low energy Lagrangian. 
I. Antoniadis and T.R. Taylor