One mismatch is discriminated in a target mRNA sequence by an inducible alkylation system based on sulfide precursors to the nucleoside 2‐amino‐6‐vinylpurine (see scheme). The reactive oligonucleotides were delivered into the cell as poly(ethylene glycol) (PEG) conjugates in polyion‐complex (PIC) micelles and showed antisense activity of high selectivity and greater potency than that of the natural antisense oligonucleotide.