Model building in type IIB string theory exhibits an attractive feature: by localizing the visible sector geometrically in the extra dimensions one may hope for a decoupling of global effects and thereby a simplified framework for extracting the predictions of the model. I will present an obstruction to this simplification which arises as a consequence of moduli stabilization. Non-perturbative effects such as gaugino condensation on a stack of D7-branes are an essential ingredient in several celebrated moduli-stabilization schemes in type IIB string theory. At the level of the four-dimensional effective theory the non-perturbative effects induce a non-perturbative superpotential and, in particular, new Yukawa interactions can be generated which are not allowed at tree-level and thus present a possible source of flavor violation. Type IIB orbifold models contain the essential features to study these non-perturbative corrections to the superpotential in string compactifications. I will describe under which conditions new Yukawa couplings arise in these settings and generalize the results to more realistic string models.
