  One of the most important and unanswered problems in particle physics is the
origin of the three generations of quarks and leptons. The standard Model does
not provide any hint regarding its sequential charge assignments, which remain
a fundamental mystery of Nature. One possible solution of the puzzle is to look
for charge assignments, in a given gauge theory, that are inter-generational,
by employing the cancellation of the gravitational and gauge anomalies
horizontally. The 331 model, based on an $SU(3)_C\times SU(3)_L \times U(1)_X$
does it in an economic way, and defines a possible extension of the Standard
Model, where the number of families has to be necessarily three. We review the
model in Frampton's formulation, that predicts the existence of bileptons.
Another characteristics of the model is to unify the $SU(3)_C\times SU(2)_L
\times U(1)_X$ into the 331 symmetry at a scale which is in the TeV range, and
can be tested at the LHC. Expressions of the scalar mass eigenstates and of the
renormalization group equations of the model are also presented.
