POU5F1 and neoplasm: Having established that GFP is expressed appropriately under the control of the endogenous Oct4 or Nanog promoters in the compound transgenic mice, we then used FACS analysis to investigate the presence of GFP+ tumor cells in primary tumors from Oct4GFP and NanogGFP MT/ret, MMTV-PyMT and MMTV-Neu mice.