The list of parties to the Nuclear Non-Proliferation Treaty encompasses the states which have signed and ratified or acceded to the international agreement limiting the spread of nuclear weapons. On 1 July 1968, the Nuclear Non-Proliferation Treaty (NPT) was opened for signature. The three depositary states were the Soviet Union (and later its successor state Russia), the United Kingdom, the United States; states wishing to become a party to the NPT must deposit their instruments of ratification, accession or succession with at least one of the depositary governments. The treaty came into force and closed for signature on 5 March 1970 with the deposit of ratification of the three depositary states and 40 others. Since then, states that did not sign the treaty may only accede to it.