  Chiral effective field theory predicts a specific charge symmetry violating
amplitude for pion production. This term is shown to provide the dominant
contribution to the forward-backward asymmetry in the angular distribution for
the reaction $pn\to d\pi^0$, for reasonable values of the mass difference
between down and up quarks, $\delta m_N$. Using a value $\delta m_N\approx$ 3
MeV leads to a prediction of a 10 standard deviation effect for a TRIUMF
experiment.
