demand the Soviet Union to stop testing. The PTBT was a first of a series of nuclear arms control treaties in the second half of 20th century. The PTBT has been considered the stepping stone to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) of 1968, which explicitly referred to the progress provided by the PTBT. In addition to the NPT, the PTBT was followed within ten years by the Outer Space Treaty and Treaty of Tlatelolco in 1967, the Seabed Arms Control Treaty in 1971, and the Anti-Ballistic Missile Treaty in 1972. In 1974, the Threshold Test Ban Treaty prohibited underground tests with yields above 150 kilotons. In October 1977, the original parties to the PTBT renewed discussion of a comprehensive test ban in Geneva. Through the end of the 1970s, the US, the UK, and Soviet Union reached agreement on draft provisions prohibiting all testing, temporarily banning PNEs, and establishing a verification system including on-site inspections. However, the sides remained at odds over the precise details of verification, and the talks would permanently disband with the departure of President Jimmy Carter in 1981. Momentum towards a comprehensive ban re-emerged under Mikhail Gorbachev and President Ronald Reagan, with Gorbachev initiating a testing moratorium in 1985. In December 1986, the US indicated support for the "long-term objective" of a comprehensive ban, followed by the commencement of testing negotiations between the US and Soviet Union in November 1987. In December 1987, the US and the Soviet Union agreed to a joint program of experiments on detecting underground tests. In August 1988, Indonesia, Mexico, Peru, Sri Lanka, Venezuela, and Yugoslavia petitioned to transform the PTBT into a comprehensive ban by extending the treaty to underground tests. At a conference on the plan in January 1991, the US indicated that it would not permit efforts to achieve a comprehensive ban by consensus with amendments to the PTBT. Throughout the 1990s, progress accelerated towards a comprehensive test ban treaty (CTBT). Following a series of international meetings on the subject, the UN General Assembly approved Resolution 50/64, which appealed for states to follow the PTBT and called for conclusion of the CTBT talks. In September 1996, the Comprehensive Nuclear-Test-Ban Treaty was signed and superseded the PTBT, but the PTBT is still in effect for states not party to the CTBT. The CTBT has yet to enter into force, as 8 required states have not