Kazutoshi Takahashi and Shinya Yamanaka demonstrated in 2006 that adult mouse cells could be reprogrammed to behave like embryonic stem cells by introducing just four transcription factors (OCT4, SOX2, KLF4, c-Myc), thereafter called the "Yamanaka factors." The discovery opened regenerative medicine without the need to destroy embryos, resolving the main ethical obstacle to stem cell research. Yamanaka received the Nobel Prize in Physiology or Medicine in 2012 together with John Gurdon. Takahashi, first author of the founding paper and the experimental executor of the discovery, was not included in the Nobel Prize.