In certain embodiments, the present invention provides oligomeric compounds having favorable toxicity profiles and therapeutic indexes. Compounds of the present invention comprise bicyclic nucleosides. Certain such bicyclic nucleosides are pyrimidines that do not include a methyl group at the 5-carbon. Oligomeric compounds comprising such nucleosides are less toxic than compounds comprising bicyclic nucleosides that do include a methyl group at the 5-carbon. In certain embodiments, the present invention provides methods of preparing and using such compounds.