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The 200-fold induction seen by transient reporter transfection of the A610-20 receptor line is likely to be an under-representation of the induction profile to be expected in double-stable lines made by selecting stably integrated inducible vectors. This is due to the tight repression that occurs for chromosomally integrated ecdysone-responsive promoters, compared with the relatively high background normally seen in transient transfection assays in which the reporter is free in the nucleus. The stable cell line ER-CHO, which was made using the receptor-expressing plasmid pERV3, shows at best 50-fold induction ratios by transient reporter transfection but the derivative double-stable line consistently gives induction ratios of 700- greater than 1,000-fold.3 Accordingly, we expect that the A610-20 receptor line will give rise to double-stable lines with induction ratios well in excess of 1,000-fold.
Retroviral delivery of the ecdysone receptors should significantly decrease
the time and labor required to screen for stable clones that express the
receptors at optimal levels, particularly for cell types that are normally
difficult to transfect. However, for difficult-to-transfect cell types, delivery
of the receptors with the pFB-ERV virus only solves half the problem. Currently,
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