have you ever used hygromycin to screen transfected HepG2 or gastric cancer cell - all the cells died, why? (Nov/19/2006 )

hi, all,
I used hygromycin B to screen stable transfected HepG2 cells and gastric cancer cell, but after I tried a serial of concentrations , from 20ug/ml to 200ug/ml, finally all the cells were dead.
I thought some cells were still alive after 25 days screening( at least they looked good), but after tripsinizing the cells , all the cells detached from the plate and looked not as good as before, I am afraid they are also going to die.
what's the problem? has hygromycin have a characteristic to make cells look like this?

you dont need to screen for 25 days. One week kill curve is more than sufficient. Why dont you go lower than 20 mcg/ml and see what happens.