  I briefly describe the recent construction of the first specific string
D-brane models yielding just the SM massless fermion spectrum. One of the most
remarkable facts predicted by these constructions is that the global symmetries
of the SM like Baryon and Lepton numbers as well as the axial $U(1)_A$ are
gauged symmetries whose anomalies are cancelled by a Green-Schwarz mechanism
only in the case of three quark-lepton generations. Proton stability and the
Dirac nature of neutrino masses are thus a prediction of these theories. This
is work done in collaboration with Fernando Marchesano and Raul Rabadan.

High Energy Physics - Phenomenology;High Energy Physics - Theory