The USPTO’s Patent Application Information Retrieval (PAIR) sytem now lists expirations due to failure to pay maintenance fees. Expired patents now have a status of "Patent Expired Due to Non-Payment of Maintenance Fees Under 37 CFR 1.362".  Patents can, of course, expire in other ways.  If a patent is subject to a terminal disclaimer relative to a previous patent and they cease to be commonly owned, for example.

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