Why cmv promoter is only for transient expression? - (Sep/09/2012 )

I was using a CMV promoter plasmid to express my protein in HEK cells. I was told that CMV promoter will slowly loose its activity and my protein expression will go down. Why is that?

Thanks!

It's usually the case that unless there is some selective pressure on the cells to maintain the promoter in an active state (e.g. antibiotic resistance), then the inserted DNA gets methylated and/or inactivated in some manner.