In quest of "just" the Standard Model on D-branes at a singularity
  In this note we explore the possibility of obtaining gauge bosons and fermionic spectrum as close as possible to the Standard Model content, by placing D3-branes at a ZN orbifold-like singularity in the presence of D7-branes. Indeed, we find that this is plausible provided a sufficiently high N is allowed for and the singular point is also fixed by an orientifold action. If extra charged matter is not permitted then the singularity should necessarily be non-supersymmetric. Correct hypercharge assignments require a dependence on some Abelian gauge D7-groups. In achieving such a construction we follow a recent observation made in Ref. [hep-th/0105155] about the possibility that, the three left handed quarks, would present different U(2) transformation properties. 
L. F. Alday and G. Aldazabal