To examine the acquisition of germ cell-like cells upon p53 deletion during cancer development, we crossed p53+/− mice with Oct4-GFP knock-in reporter mice, which harbor IRES-GFP fusion cassette downstream of the stop codon of the Oct4 (Pou5f1) gene, and the resulting mice were further intercrossed to generate p53+/+/Oct4-GFP+/+ and p53−/−/Oct4-GFP+/+ mice for further study (Fig. 1a). The gene discussed is POU5F1; the disease is cancer.