urgent: loop or no loop? - negative control (Feb/15/2006 )

I'm using the pRNAT 3.1.CMV vector for siRNA in human glioma cell lines. The oligo I am using form a hairpin (structure of oligo: sense sequence-loop-antisense sequence). My question is: must the negative control also have the same structure, or is it sufficient if it is only a nonsense sequence, which does not form a hairpin?

loop necessary for good noegative control.
If your transcript is not capable to form an active structure it's not a negative control.