YAC arm stability without selection?

[Anthony Tomlinson]

We have a pYAC4 derived YAC, and we would like to delete
the URA3 marker on the right arm so that we can use URA3
to replace a gene within the YAC.  Has anyone out there tried
growing pYAC4 derived YACs without selection for one or
both arms?  How stable is it?

Thanks,  

Anthony