Witten's mechanism in the flipped SU(5) unification

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.