Neutron-anti-neutron oscillation as a test of grand unification
  We discuss the predictions for neutron-anti-neutron oscillation in various supersymmetric and non-supersymmetric grand unified theories. It is pointed out that in a new class of superstring inspired grand unified theories of $E_6$-type that satisfy gauge coupling unification, breakdown of B-L symmetry occurs at an intermediate scale leading in turn to $\Delta B=1$ type R-parity violating interactions naturally suppressed to the level of $10^{-5}$ to $10^{-7}$. This inturn implies an $N-\bar{N}$ transition time of order $10^{10}$ to $10^{11}$ sec. which may be observable in the next generation of experiments. This model also satisfies the conditions needed for generating the cosmological baryon asymmetry of the right order of magnitude for a restricted range of the parameter space. 
R. N. Mohapatra