What is the best location on an siRNA duplex to incorporate a dye label?

Modification at the 5'-end of the antisense strand has been shown to interfere with siRNA silencing activity and therefore this position is not recommended for modification. Modifications at the other three termini have been shown to have minimal to no effect on silencing activity. Modification on the 5'-end of the sense strand is the position that provides the most efficient chemical synthesis and thus is recommended.