Assignor estoppel
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Assignor estoppel is the patent law doctrine that an assignor of a patent is estopped, or barred, from later contesting the validity of the patent. The logic behind this doctrine is that by assigning the patent, the patent-holder accepted its validity, and cannot contradict himself on that point later.
The America Invents Act abrogated this doctrine for inter partes proceedings,[1] however, which further undermines the validity of patents like much of what the AIA has done.