Nambu mass hierarchies in strings
  We show that a recent proposal by Nambu to generate a hierarchy among Yukawa couplings in the standard model may be easily implemented in superstring models. In such models, two of the main ingredients of the Nambu proposal find a natural explanation: minimising with respect to the Yukawas amounts to a minimisation with respect to the underlying moduli fields and a constraint on the Yukawas of the type of the Veltman condition may be attributed to the relaxation process to a phenomenologically viable string vacuum. 
P. Binetruy and E. Dudas