  Intersecting D-brane models provide an attractive explanation of family
replication in the context of string theory. We show, however, that the
localization of fermion families at different brane intersections in the extra
dimensions introduces flavour changing neutral currents mediated by the
Kaluza-Klein excitations of the gauge fields. This is a generic feature in
these models, and it implies stringent bounds on the mass of the lightest
Kaluza-Klein modes (becoming severe when the compactification radii are larger
than the string length). We present the full string calculation of four-fermion
interactions in models with intersecting D-branes, recovering the field theory
result. This reveals other stringy sources of flavour violation, which give
bounds that are complementary to the KK bounds (i.e. they become severe when
the compactification radii are comparable to the string length). Taken together
these bounds imply that the string scale is larger than $M_s\gtrsim 10^2$ TeV,
implying that non-supersymmetric cases are phenomenologically disfavoured.

High Energy Physics - Phenomenology;High Energy Physics - Theory