  High-resolution CO maps of 9 molecular clouds in IC 10 are combined with the
new measurement of the distance to this nearby metal poor galaxy to measure
accurately the CO-to-H2 conversion factor. The result for IC 10 is combined
with published data for four other Local Group galaxies (M31, M33, NGC 6822,
the SMC) to trace the dependence of the CO-to-H2 conversion factor on oxygen
abundance. These data show conclusively that the CO-to-H$_2$ conversion factor
increases as the metallicity of the host galaxy decreases, with the conversion
factor increasing by a factor of 4.6 for a factor of 10 decrease in
metallicity.
