The W88 is a United States thermonuclear warhead, with an estimated yield of 475 kilotons (kt), and is small enough to fit on MIRVed missiles. The W88 was designed at the Los Alamos National Laboratory in the 1970s. In 1999, the director of Los Alamos who had presided over its design described it as "the most advanced U.S. nuclear warhead". As of 2021, the latest version is called the W88 ALT 370, the first unit of which came into production on 1 July, 2021, after 11 years of development. The Trident II submarine-launched ballistic missile (SLBM) can be armed with up to 8 W88 warheads (Mark 5 re-entry vehicle) or 12 100 kt W76 warheads (Mark 4 re-entry vehicle), but it is limited to 8 warheads under the Strategic Offensive Reductions Treaty.