A Three-Family $SU(4)_c \otimes SU(2)_w \otimes U(1)$ Type I Vacuum
  We construct a four dimensional chiral N=1 space-time supersymmetric perturbative Type I vacuum corresponding to a compactification on a toroidal Z_2 X Z_2 X Z_3 orbifold with a discrete Wilson line. This model is non-perturbative from the heterotic viewpoint. It has three chiral families in the SU(4)_c X SU(2)_w X U(1) subgroup of the total gauge group. We compute the tree-level superpotential in this model. There appears to be no obvious obstruction to Higgs the gauge group down to SU(3)_c X SU(2)_w X U(1)_Y and obtain the Standard Model gauge group with three chiral families. 
Zurab Kakushadze