Under the U.S. patent law, the assignor estoppel doctrine means that the assignor who has assigned patent rights to the assignee cannot claim that the patent rights is invalid in a patent infringement lawsuit after the assignment of the patent right. The doctrine applies even if the assignor assigns the patent application. The doctrine is based on the “fair dealing” or “principle of fairness” between the assignor and the assignee regarding the assignment of the patented invention. In addition, the doctrine has its origins of the principle under the real estate property law called the “estoppel by deed” that the assignor cannot claim that the deed of title containing the valid right to the real estate is invalid. The U.S. Supreme Court recognized the doctrine as a principle of the U.S. patent law through Westinghouse decision in 1924. However, since the Westinghouse decision did not apply the doctrine, and the subsequent Scott Paper decision in 1945 also did not apply the doctrine, many questions has been raised as to whether the doctrine is valid as a principle of the U.S. patent law. However, the Diamond Scientific decision decided in 1988 after the CAFC was established, strengthened the legal status of the doctrine, and the CAFC expanded the scope of this principle. On June 29, 2021, the Supreme Court again decided whether to abolish the doctrine through the Minerva decision, and reconfirmed that the doctrine is valid as a principle of the U.S. patent law. However, the Supreme Court explicitly ruled that, unlike the principle of the CAFC, there is a limit to the scope of the doctrine. In this respect, the Minerva decision can be evaluated as the decision that weakened the effects of patent rights, along with many decisions of the Supreme Court since the 2000s.