Loss of silencing activity caused by 5＇-terminal modification with D-/L-isonucleotide （isoNA） or locked nucleic acid （LNA） could not be restored by 5＇-terminal phosphorylation

In this study, a series of 5＇-phosphorylated siRNAs with D-/L-isonucleotide （isoNA） or locked nucleic acid （LNA） incorporated at the 5＇-terminus were synthesized. It was found that after incorporating either isoNA or LNA at the 5＇-terminus of the anti sense strand or sense strand, the silencing activity of modified strands has been inhibited, which cannot be recovered by phos- phorylation at the 5＇-terminus; however, the silencing activity of unmodified strand to its own target was increased. This work indicates that the isoNA and LNA modification at 5＇-terminus can interfere with the strand selection during the RISC assem- bling process, and the disturbance of the 5＇-phosporylation should not be the only viable mechanism.