describe most of the observed features or charge symmetry breaking in nuclear physics. While it is certainly true that other mechanism cannot be ruled out, $\rho$-$\omega$ mixing appears to give a consistent description of the bulk of the experimental data.

Recently, this success has been called into question. The momentum dependence of the $\rho-\omega$ mixing amplitude has been calculated using several different models[@GHT92; @PW92; @KTW93; @MTRC94]. While these models are based on quite different physical assumptions, they all share one important quality: the $\rho-\omega$ mixing at spacelike momenta in all of these models is quite different from its value at the $\omega$ pole—generally of the opposite sign and significantly reduced in magnitude. Indeed it has been shown that for a wide class of models[@OCPTW94] the mixing must go to zero at $q^2=0$ implying that amplitude changes sign. Moreover, a QCD sum-rule calculation, also apparently gives a similarly large momentum dependence of the coupling[@HHKM94]. Since the N-N potential probes the spacelike region, this appears to imply that the vector meson exchange part of the charge symmetry breaking nucleon-nucleon NN potential is very different from one based on the on-mass-shell mixing. Indeed, NN potentials have been constructed based on these momentum dependent 