What is Nuclear Non-Proliferation Treaty that Iran is considering exiting?

Nuclear Non-Proliferation Treaty, in force since 1970 with 190 members, aims to prevent the spread of nuclear weapons. It recognises five nuclear-weapon states—USA, Russia, UK, France, China. Four nuclear-armed countries—India, Pakistan, Israel, North Korea—aren't a part. Iran is considering exiting as US-Israel attacked its nuclear sites, negating its right to develop peaceful nuclear energy.