MSSM via Pati-Salam from Intersecting Branes on $T^6/(\mathbf{\Z_2}   \times \mathbf{\Z_2'})$
  We construct an MSSM-like model via Pati-Salam from intersecting D-branes in Type IIA theory on the $\Z_2 \times \Z_2'$ orientifold where the D-branes wrap rigid 3-cycles. Because the 3-cycles are rigid, there are no extra massless fields in the adjoint representation, arising as open-string moduli. The presence of these unobserved fields would create difficulties with asymptotic freedom as well as the prediction of gauge unification. The model constructed has four generations of MSSM matter plus right-handed neutrinos, as well as additional vector-like representations. In addition, we find that all of the required Yukawa couplings are allowed by global symmetries which arise from U(1)'s which become massive via a generalized Green-Schwarz mechanism. Furthermore, we find that the tree-level gauge couplings are unified at the string scale. 
Ching-Ming Chen, V.E. Mayes, D.V. Nanopoulos