POU5F1 and cancer: Oct4 was expressed early in the preimplantation embryo and thus regulated early events of murine development.53,54 Oct4 was first shown to be associated with cancer by M Monk in 2001.55 In 2005, based on CSCs in breast cancer, Ponti and colleagues56 found that the CD44+CD24- fraction expressed Oct4 and gave rise to new tumors.