The doctrine of assignor estoppel is a doctrine of United States patent law

United States patent law

United States patent law was established "to promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;" as provided by the United States Constitution. Congress implemented these...

 barring a patent

Patent

A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

's seller (assignor) from attacking the patent's validity in subsequent patent infringement litigation. The doctrine is based on the doctrine of legal estoppel

Legal estoppel

Legal estoppel is a principle of law, particularly United States patent law, that an assignor or grantor is not permitted subsequently to deny the validity of title to the subject matter of the assignment or grant...

, which prohibits a grantor (typically, of real property) from challenging the validity of his/her/its grant.

In Diamond Scientific Co. v. Ambico, Inc., the United States Court of Appeals for the Federal Circuit distinguished the policies applicable to assignor estoppel from those applicable to licensee estoppel

Licensee estoppel

Licensee estoppel was a U.S. patent law doctrine, now overturned, that a licensee under a patent would not be permitted to challenge the validity of the patent. The Supreme Court, in Lear, Inc. v...

. It therefore held that the doctrine of Lear, Inc. v. Adkins

Lear, Inc. v. Adkins

Lear, Inc. v. Adkins, 395 U.S. 653 , is a decision of the U.S. Supreme Court overturning the doctrine of licensee estoppel and holding that public interest considerations require that licensees be free to challenge the validity of possibly spurious patents under which they are licensed. This...

, which applies to licenses and holds that public policy requires that licensees not be muzzled from challenging the validity of possibly spurious patents, does not apply to assignments.

The UK counterpart to this doctrine is the doctrine of non-derogation from grants

Doctrine of non-derogation from grants

The doctrine of non-derogation from grants is a principle of the law of England and Wales. As the House of Lords explained in British Leyland Motor Corp. v...

. Under this doctrine, as explained in British Leyland Motor Corp. v. Armstrong Patents Co.

British Leyland Motor Corp. v. Armstrong Patents Co.

British Leyland Motor Corp. v. Armstrong Patents Co. is a 1986 decision of the House of Lords concerning the doctrine of non-derogation from grants. This doctrine is comparable to, but somewhat broader than, the doctrine of legal estoppel, assignor estoppel, or estoppel by deed in U.S. law...

, a seller of realty or goods is not permitted to take any action (such as bringing an infringement action) that will lessen the value to the buyer of the thing sold. Thus, the owner of copyright in the tailpipe of a motor car, having sold the motor car, may not then bring a copyright infringement action to prevent the aftermarket sale of replacement tailpipes to purchasers of those motor cars.

Examples

• For example, if Tom sold his U.S. patent rights to Jerry, and was sued by Jerry over infringement of that patent
  Patent infringement
  Patent infringement is the commission of a prohibited act with respect to a patented invention without permission from the patent holder. Permission may typically be granted in the form of a license. The definition of patent infringement may vary by jurisdiction, but it typically includes using or...
  
   later, Tom is not allowed to challenge the patent's validity because he was the inventor.

"Doctrine of assignor estoppel prevents unfairness and injustice of permitting party to sell patent rights and later assert that what he sold is worthless." Mentor Graphics Corp. v. Quickturn Design Systems, Inc., 150 F.3d 1374 (Fed. Cir. 1998).

• This doctrine extends to those in "privity" with the assignor. If Tom becomes another company's largest shareholder, and that company is sued by Jerry over patent infringement, that company could very likely be barred from raising patent validity as a defense even if Tom was not personally involved in the infringing process.

Case law

• Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266 U.S. 342, 45 S.Ct. 117 (1924).
• Diamond Scientific Co. v. Ambico, Inc., 848 F.2d 1220 (Fed. Cir. 1988), in some circumstances, equity may outweigh the estoppel doctrine.