  The spontaneous muonium to antimuonium conversion is one of the interesting
charged lepton flavor violation processes. It serves as a clear indication of
new physics and plays an important role in constraining the parameter space
beyond Standard Model. MACE is a proposed experiment to probe such a phenomenon
and expected to enhance the sensitivity to the conversion probability by more
than two orders of magnitude from the current best upper constraint obtained by
the PSI experiment two decades ago. Recent developments in the theoretical and
experimental aspects to search for such a rare process are summarized.
