The Third Family is Different
  It now appears phenomenologically that the third family of fundamental fermions may be essentially different fron the first two. Particularly the high value (174GeV?) of the top quark mass suggests a special role. In the standard model all three families are treated similarly [becoming exactly the same at asymptotically high energies] so we need to extend the model to accommodate the goal of a really different third family. In this article I describe not one but two such viable extensions, quite different one from another. The first is the 331 model which predicts dileptonic gauge bosons. The second is the $Q_6$ model which predicts additional leptons between 50 and 200 GeV. One expects there are many other models of this general type characterized by the prediction of new particles at accessible masses. Supersymmetrization will not be discussed here. 
Paul H. Frampton