This invention provides compounds, compositions and methods for modulating the expression of human p21 using RNA interference. The RNA interference molecules can be used in methods for preventing or treating diseases such as malignant tumor. Provided are a range of siRNA structures, having one or more nucleotides being modified or chemically-modified. Advantageous structures include siRNAs with 2′-deoxy nucleotides located in the seed region, as well as other nucleotide modifications.