Instintens, anomalies and SU(5)

For d = 10 gauge theories one has vacuum tunnelling caused by d = 10 instantons, or ''instintens'', provided the gauge group is SU(n) n>=5, SO(10), or E6. The associated theta-angle is a physical marameter in the supersymmetric d = 10 Yang-Mills theory despite the presence of a massless fermion. It is likely that d = 10 super Y-M theory is consistent only for the groups SU(2), SO(5), E8. In this case, the requirement of non-trivial topology and consistency leads uniquely to the choice of gauge group SU(5)