  We report on progress towards the construction of SM-like gauge theories on
the world-volume of D-branes at a Calabi-Yau singularity. In particular, we
work out the topological conditions on the embedding of the singularity inside
a compact CY threefold, that select hypercharge as the only light U(1) gauge
factor. We apply this insight to the proposed open string realization of the SM
of hep-th/0508089, based on a D3-brane at a dP_8 singularity, and present a
geometric construction of a compact Calabi-Yau threefold with all the required
topological properties. We comment on the relevance of D-instantons to the
breaking of global U(1) symmetries.
