stable cell line - (Apr/13/2009 )

Hi,
I'm trying to create stable cell lines using pBudCE4.1 expression vector. I isolated several colonies. My problem is I don't when to give up using zeocin. (the vector has zeocin resistant gene on it) My other question is if I cannot see any expression in mRNA level, does it mean that I don't need to check it for protein level?

Once you obtained positive clones, the stable lines should still be maintained in the selection medium with the antibiotics concentration reduced to half.

If there is no increase in mRNA expression, there should be no protein too.