From $m_{d}=m_{e}$ to Realistic Mass Relations in Quark-Lepton Symmetric   Models
  In recent years a new potential symmetry of fundamental particle physics has been investigated --- discrete quark-lepton symmetry. When this symmetry is implemented, however, it often leads to either of the unrealistic predictions $m_{u}=m_{e}$ or $m_{d}=m_{e}$. This paper considers two possible ways models based on $m_{d}=m_{e}$ can be made realistic. 
D. S. Shaw and R. R. Volkas