  We argue that the Witten's loop mechanism for the right-handed Majorana
neutrino mass generation identified originally in the SO(10) grand unification
context can be successfully adopted to the class of the simplest flipped SU(5)
models. In such a framework, the main drawback of the SO(10) prototype, in
particular, the generic tension among the gauge unification constraints and the
absolute neutrino mass scale is alleviated and a simple yet potentially
realistic and testable scenario emerges.
