inducible shRNA promoter leaking - (Jan/27/2007 )

Hello, Iam trying to knock down a gene which is necessary for long term viability of the cell lines. I tried to establish a stable cell line using
Hela cells expressing tet repressor and pRNATin inducible vector with GFP marker. But after selection, I had only 3 green clones, with no functional knockdown.
When I used empty vector, selection was OK- lots of green clones.

Has anyone suggestions about some better inducible shRNA/miRNA expression system?

a recent publication showed an inducible H1 promoter by tetracycline element.

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