The supersymmetric standard model from the Z_6' orientifold?
  We construct N=1 supersymmetric fractional branes on the Z_6' orientifold. Intersecting stacks of such branes are needed to build a supersymmetric standard model. If a,b are the stacks that generate the SU(3)_c and SU(2)_L gauge particles, then, in order to obtain just the chiral spectrum of the (supersymmetric) standard model (with non-zero Yukawa couplings to the Higgs multiplets), it is necessary that the number of intersections a \circ b of the stacks a and b, and the number of intersections a \circ b' of a with the orientifold image b' of b satisfy (a \circ b, a \circ b')=\pm(2,1) or \pm(1,2). It is also necessary that there is no matter in symmetric representations of the gauge group, and not too much matter in antisymmetric representations, on either stack. We provide a number of examples having these properties. Different lattices give different solutions and different physics. 
David Bailin, Alex Love