Multiple mutations contribute to establish cancers. We have searched for potential oncogenes by screening cDNA libraries derived from gastric cancer cell lines, pancreatic cancer cell lines and glioma cell lines, using retrovirus-mediated expression cloning. Two types of interleukin-3 (IL-3)-dependent cell lines, Ba/F3 and HF6, were transduced with the cDNA libraries and several genes that render these cells factor-independent were identified including PIM-1, PIM-2, PIM-3, GADD45B and reproductive homeobox genes on the X chromosome gene F2 (RHOXF2). Although no mutation in these genes was found, these molecules were highly expressed in cancer cell lines and they may play important roles in cell transformation. Among them, we focused on a transcriptional repressor RHOXF2. Transduction of RHOXF2 rendered HF6 cells factor-independent, while knockdown of RHOXF2 inhibited growth of the HGC27 gastric cancer cell line which highly expresses RHOXF2. In addition, RHOXF2-transduced HF6 cells quickly induced leukemia when transplanted into sublethally irradiated mice. Moreover, RHOXF2 is highly expressed in some leukemia cell lines and a variety of human cancer samples including colon and lung cancers. Thus, these results indicate that RHOXF2 is involved in carcinogenesis.