  The ultimate dream of unification models consists in combining both gauge and
Yukawa couplings into one unified coupling. This is achieved by using a
supersymmetric exceptional ${\rm E}_6$ gauge symmetry together with asymptotic
unification in compact five-dimensional space-time. The ultraviolet fixed point
requires exactly three fermion generations: one in the bulk, and the two light
ones localised on the ${\rm SO}(10)$ boundary in order to cancel gauge
anomalies. A second option allows to preserve baryon number and to lower the
compactification scale down to the typical scales of the intermediate
Pati-Salam gauge theory.
