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US patent law lacks the robust post-grant opposition procedure available to third-party challengers under the European Patent Convention. Instead, US law offers two variants of post-issuance re-examination proceeding, each of which permits limited third-party challenge to patent validity. Neither is widely used, but ex parte re-examination, in which the third party is precluded from participating in the proceedings after filing the initial re-examination request, has proven to have some value as a tool in defence of actual or threatened infringement litigation.
