(A) Effect of abasic modifications in positions 9–12 of the sense strand of siRNA-1; (B) effect of 2′-O-methyl and/or phosphorothioate modifications between positions 9 and 10 of the sense strands of siRNA-1 and siRNA-2; changes in potency relative to the parent siRNA are expressed as IC₅₀ (parent)/IC₅₀ (modified); P = S, phosphorothioate linkage between positions 9 and 10; g, 2′-O-methylguanosine.