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We provide a general overview of the current state of the art in three generation model building proposals - using intersecting D-brane toroidal compactications of IIA string theories - which have, only, the SM at low energy. In this context, we focus on these model building directions, where natural non-supersymmetric constructions based on SU(4)C SU(2)L SU(2)R, SU(5) and flipped SU(5) GUT groups, have at low energy only the Standard Model. In the flipped SU(5) GUTS, the special build up structure of the models accommodates naturally a see-saw mechanism and a new solution to the doublet-triplet splitting problem.

4D GUT (AND SM) MODEL BUILDING FROM INTERSECTING D-BRANES