Towards classification of SO(10) heterotic string vacua

We report some recent progress towards classification of phenomenologically appealing heterotic string models in the Free Fermionic Formulation. We focus on a class of Z2xZ2 models with SO(10) space-time gauge symmetry and study their main phenomenological aspects. We further consider reducing the gauge symmetry to the Pati-Salam gauge group SU(4)XSU(2)_LxSU(2)_R and impose a series of phenomenological constraints including the existence of gauge symmetry breaking Higgs particles and the elimination of exotic fractionally charged states.