Small interfering RNA (siRNA) molecules that target a mammalian ribonucleotide reductase gene, and which are capable of inhibiting the expression of their target gene are provided. The siRNA molecules of the invention are capable of attenuating neoplastic cell growth and/or proliferation in vitro and in vivo and, therefore, can be used to attenuate the growth and/or metastasis of various types of mammalian cancers.