Good examples of the causal effects of homeobox genes were obtained in animal models with Oct-4, which promoted testicular germ cell tumors when expressed inappropriately [18]; NKX3.1, which induced prostatic intraepithelial neoplasia and enhanced prostate cancer progression in collaboration with loss of function of the PTEN tumor-suppressor gene [19]; and SIX1, the overexpression of which induced breast cancer in mammary epithelial cells [20]. The gene discussed is POU5F1; the disease is testicular germ cell tumor.