1. F. Wilczek, Axions and family symmetry breaking, Phys. Rev. Lett. 49 (1982) 1549 [ SPIRES ].
2. R.N. Mohapatra, Gauge model for chiral symmetry breaking and muon electron mass ratio, Phys. Rev. D 9 (1974) 3461 [ SPIRES ].
3. S.M. Barr and A. Zee, Calculating the electron mass in terms of measured quantities, Phys. Rev. D 17 (1978) 1854 [ SPIRES ].
4. C.L. Ong, Adding a horizontal gauge symmetry to the Weinberg-Salam model: an eight quark model, Phys. Rev. D 19 (1979) 2738 [ SPIRES ].
5. F. Wilczek and A. Zee, Horizontal interaction and weak mixing angles, Phys. Rev. Lett. 42 (1979) 421 [ SPIRES ].