Stable cell line losing expression faster than usual - (Aug/26/2007 )

Hi, I have a HEK293 stable cell line maintained in 400microgram/ml G418. From the frozen aliquots, the expression of the gene is consistently lost after 2.5-3 months (the gene slows down growth) after being brought up. However, with the latest aliquot, the expression was lost after merely 6 weeks. Just wondering what could have contributed to the accelerated loss of expression? Thanks

well your G418 solution may not be fresh... or you ay have change of serum ?

Thanks for the reply. I keep my G418 solution in the fridge, usually for no less than 2 weeks. I did change serum though, but how would that affect the expression? If I switch to the previous type (New Zealand Vs Australia), but not necessarily the same batch, can I expect a longer expression period? Thanks

Correction: I found out I did not change serum. So back to square one...

Are the latest aliquot and the previous ones from the same batch, i.e. frozen at the same time? How long did you left it frozen already? In liquid N2 or just -80oC?

Yes, they are from the same batch, which were frozen down in LN2 about 1 year ago.

How do you identify your HEK293 stable cell line? Is your expression vector remained as episome or would it be integrated to the cell genome?