Max. length for 3'UTR in luciferase assay? - (Jul/20/2009 )

Hello everybody.
What do you think is the maximal length possible of a 3'UTR to be cloned into a luciferase vector for target validation? Or what are the advantages/disadvantages of certain lengths? Just to clone the specific target sequence I consider too artificial.
Thank you for any help.

I think it is up to the capacity of the luciferase vector you use. But bear in mind, the longer the 3'UTR, the more difficult it is to PCR amplifying the sequence and the more likely you get mutations.