Most human tumors harbor multiple genetic alterations, including dominant mutant oncogenes. It is often not clear which of these oncogenes are continuously required and which, when inactivated, may inhibit tumorigenesis. Recently, we developed a vector that mediates suppression of gene expression through RNA interference. Here, we use a retroviral version of this vector to specifically and stably inhibit expression of only the oncogenic K-RAS^V12 allele in human tumor cells. Loss of expression of K-RAS^V12 leads to loss of anchorage-independent growth and tumorigenicity. These results indicate that viral delivery of small interfering RNAs can be used for tumor-specific gene therapy to reverse the oncogenic phenotype of cancer cells.