  We do not know why there are three fermion families in the Standard Model
(SM), nor can we explain the observed pattern of fermion masses and mixing
angles. Standard grand unified theories based on the SU(5) and SO(10) groups
fail to shed light on this issue, since they also contain three copies of
fermion representations of an enlarged gauge group. However, it does not need
to be so: the Standard Model families might be distributed over distinct
representations of a grand unified model, in which case the gauge symmetry
itself might discriminate the various families and explain (at least partially)
the flavor puzzle. The most ambitious version of this idea consists on
embedding all SM fermions in a single irreducible representation of the gauge
group.
