The present invention concerns an isolated siRNA of from about 5 to about 20 nucleotides that mediates RNA interference. Also disclosed are methods of reducing expression of a target gene in a cell comprising obtaining at least one siRNA of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 basepairs in length; and delivering the siRNA into the cell. The siRNAs can be chemically synthesized RNA or an analog of a naturally occurring RNA.