  A common way to obtain standard-model-like Lagrangians in string theory is to
place D3-branes inside flux compactifications. The bosonic and fermionic masses
and couplings of the resulting gauge theory are determined by the
ten-dimensional metric and the fluxes, respectively, and the breaking of
supersymmetry is soft. However, not any soft-supersymmetry-breaking Lagrangian
can be obtained this way since the string theory equations of motion impose
certain relations between the soft couplings. We show that for D3-branes in
background fluxes, these relations imply that the sums of the squares of the
boson and of the fermion masses are equal and that, furthermore, one- and
two-loop quantum corrections do not spoil this equality. This makes the use of
D3-branes for constructing computationally controllable models for physics
beyond the standard model problematic.
