  We consider the most general generation-independent U(1) gauge symmetry
consistent with the presence of Yukawa couplings for all quarks and leptons in
the SUSY version of the Standard Model. This U(1) has generically mixed
anomalies with SM groups, which cannot be cancelled by the Green-Schwarz
mechanism of heterotic D=4 strings. We argue that these anomalies can in
principle be cancelled by the generalized Green-Schwarz mechanism present in
field theories corresponding to D-branes at singularities. Moreover, unlike the
heterotic case, once the U(1) symmetry is broken it may remain as an exact
perturbative global symmetry in the low energy theory. Applying this scheme to
the SUSY SM we find that gauging such a general U(1):1) B and L violating
operators at least up to dim=3,4,5,6 are generically forbidden; 2)The
$\mu$-term is generically supressed. We also study the properties of a U(1)
symmetry whose mixed anomalies with the different SM gauge groups are in the
ratio of the beta function coefficients $\beta_a$. This relation has been shown
to hold in certain orientifold models. In all cases the U(1) remains as a
global symmetry at the orientifold singularity, the SM Higgs can break it at
the electroweak scale, making possible to relate the blowing-up of the
singularity with electroweak symmetry breaking.
