  We discuss a new mechanism in which non-perturbative quantum gravity effects
directly generate a Majorana mass for the neutron. In particular, in string
theory, exotic instantons can generate an effective six quark operator by
calculable mixed disk amplitudes. In a low string scale scenario, with
$M_{S}\simeq 10\div 10^{5}\, \rm TeV$, a neutron-antineutron oscillation can be
reached in the next generation of experiments. we argue that protons and
neutralinos are not destabilized and that dangerous FCNCs are not generated. We
show an example of quiver theories, locally free by tadpoles and anomalies,
reproducing MSSM plus a Majorana neutron and a Majorana neutrino. These models
naturally provide a viable baryogenesis mechanism by resonant RH neutrino
decays, as well as a stable WIMP-like dark matter. We also discuss how LHC data
will provide useful inputs for our model, in the immediate future, testing
several different regions of the parameter space. Such a mechanism represents a
simple and calculable counter-example to the Wilsonian UV completion of
effective non-renormalizable operators.
