One aspect of the present invention relates to a double-stranded oligonucleotide comprising at least one non-natural nucleobase. In certain embodiments, the non-natural nucleobase is difluorotolyl, nitroindolyl, nitropyrrolyl, or nitroimidazolyl. In a preferred embodiment, the non-natural nucleobase is difluorotolyl. In certain embodiments, only one of the two oligonucleotide strands comprising the double-stranded oligonucleotide contains a non-natural nucleobase. In certain embodiments, both of the oligonucleotide strands comprising the double-stranded oligonucleotide independently contain a non-natural nucleobase. In certain embodiments, the oligonucleotide strands comprise at least one modified sugar moiety. Another aspect of the present invention relates to a single-stranded oligonucleotide comprising at least one non-natural nucleobase. In a preferred embodiment, the non-natural nucleobase is difluorotolyl. In certain embodiments, the ribose sugar moiety that occurs naturally in nucleosides is replaced with a hexose sugar, polycyclic heteroalkyl ring, or cyclohexenyl group. In certain embodiments, at least one phosphate linkage in the oligonucleotide has been replaced with a phosphorothioate linkage.