  We review two systematic bottom-up analyses of MSSM quivers recently
performed by the authors. We extend the analysis of arXiv:0905.3379 by
including constraints arising from proton decay via dimension 5 operators and
present all four-stack quivers in the Madrid embedding which satisfy this
additional constraint. Furthermore, we investigate and make precise the
interplay between mass hierarchies obtained via factorizable Yukawa textures
and the presence of dimension 5 proton decay operators in MSSM orientifold
compactifications. We discuss this issue in a five-stack quiver, first
presented in arXiv:0909.4292, which exhibits proper mass hierarchies and no
rapid proton decay.
