Charge quantization in the largest leptoquark-bilepton chiral   electroweak scheme
  The uniqueness of the hypercharge assignments in the three fermion families leptoquark-bilepton $SU(3)_C \times SU(4)_L \times U(1)_N$ model is established. Although the gauge group contains an explicit U(1) factor, freedom from triangle anomalies combined with the requirement of nonvanishing charged fermion masses uniquely fix the electric charges of all fermions independently of the neutrinos being massless or not. The electric-charge quantization, family replication, and the existence of three colors are interwoven. 
A. Doff and F. Pisano