One aspect of the present invention relates to an oligonucleotide agent comprising at least one universal nucleobase. In certain embodiments, the universal nucleobase is difluorotolyl, nitroindolyl, nitropyrrolyl, or nitroimidazolyl. In a preferred embodiment, the universal nucleobase is difluorotolyl. In certain embodiments, the oligonucleotide is double-stranded. In certain embodiments, the oligonucleotide is single-stranded. Another aspect of the present invention relates to a method of altering the expression level of a target in the presence of target sequence polymorphism. In a preferred embodiment, the oligonucleotide agent alters the expression of different alleles of a gene. In another preferred embodiment, the oligonucleotide agent alters the expression level of two or more genes. In another embodiment, the oligonucleotide agent alters the expression level of a viral gene from different strains of the virus. In another embodiment, the oligonucleotide agent alters the expression level of genes from different species.