I have a simple question regarding masses of elementary particles (in string theory). What is the mechanism proposed to explain the tiny but non-zero fermion masses? I know zero masses e.g. due to some symmetry (gauge, conformal) or symmetry breaking (Goldstone bosons). I know the huge masses at and beyond Planck-scale due to higher modes of the string. I know mass generation due to non-linear interaction like for hadron masses in QCD. But all these mechanism do not seem applicable for the generation of fermion masses. Of course one could assume a very complicated self-interaction a la Higgs and try to fit a potential that reproduces all these masses; but that seems to be unsatisfying, doesn't it?