  Nontrivial twisted boundary conditions associated with extra compact
dimensions produce an ambiguity in the value of the four dimensional coupling
constants of the renormalizable interactions of the twisted fields' zero modes.
Resolving this indeterminancy would require a knowledge of the exact form of
the higher dimensional action including the coefficients of higher dimensional
operators. For the case of moderately sized extra dimensions, the uncertainty
in the coupling constants can be of order one and may lead to modifications in
the stability of the model.

High Energy Physics - Theory