The present invention provides a method for identifying a gene which is expressed at a modulated level in an aggressive cancer by: (a) measuring baseline expression levels of genes in cancer cells; (b) introducing into the cancer cells an expression vector which directs expression of rat PEG-3; (c) measuring the expression levels of genes in the cells from step (b); and (d) identifying genes, the expression of which is modulated in an aggressive cancer. In another aspect of this invention, the measurement of expression levels of genes in step (a) and/or step (c) comprises measuring mRNA levels. In yet another aspect of the invention, the measurement of expression levels of genes in step (a) and/or step (c) comprises measuring RNA levels. In another aspect of the invention, the genes identified in step (d) are used for diagnosing whether a subject suffers from an aggressive form of cancer.