  We study a very simple, yet potentially realistic renormalizable flipped
SU(5) scenario in which the right-handed neutrino masses are generated at very
high energies by means of a two-loop diagram similar to that identified by E.
Witten in the early 1980's in the SO(10) GUT framework. This mechanism leaves
its traces in the baryon number violating signals such as the proton decay,
especially in the "clean" channels with a charged lepton and a neutral meson in
the final state.
