Aspects of the invention include methods of reducing the deleterious activity of a mutant extended nucleotide repeat (NR) containing target gene in a cell by contacting the cell with an effective amount of a nucleoside agent, as well as compositions used in such methods. The deleterious activity (e.g., toxicity and/or dis-functionality of products encoded thereby) of a mutant extended NR containing target gene may be reduced in a variety of different ways, e.g., by reducing (and in some instances differentially, including selectively, reducing) the production or activity of toxic expression products (e.g., RNA or protein) encoded by the target gene. Kits and compositions for practicing the subject methods are also provided.