On Tuesday, the Supreme Court invalidated a patent that effectively claimed ownership of a fact about the human body. A joint brief by the Cato Institute, Competitive Enterprise Institute, and the Reason Foundation had urged the high court to reject the patent, arguing that it posed a threat to freedom of thought and innovation.


The patent focused on a class of drugs called thiopurines, which are used to treat autoimmune diseases. When a patient takes a thiopurine drug, it is processed by the body into chemicals known as “metabolites.” Doctors measure metabolite levels in order to properly adjust the dosage of thiopurine drugs.


A company called Prometheus Labs filed a patent on a thiopurine drug testing process. The patent didn’t cover the drugs themselves or any particular method for measuring metabolite levels—these had already been invented years ago by other companies. Rather, Prometheus patented the idea that particular metabolite levels “indicate a need” to raise or lower the drug dosage. In effect, Prometheus was claiming ownership of a basic fact about the human body.


When the Mayo Clinic created its own thiopurine testing product, Prometheus sued. It argued that when a doctor used Mayo’s test, she could infringe Prometheus’s patent by thinking about the scientific correlation is disclosed. And that, in turn, made Mayo an accessory to the doctor’s infringing thoughts.


But Mayo argued that the patent was invalid because it claimed a law of nature, something the Supreme Court has said repeatedly isn’t eligible for patent protection. Mayo’s argument was supported not only by the Cato Institute, but also by the American Civil Liberties Union, the American Medical Association, the American Association of Retired Persons, and many other groups. They warned that it was dangerous to grant patents that can be infringed by mere thoughts, and that such patents would harm the quality of medical care by restricting doctors’ access to information.


In an unanimous decision by Justice Stephen Breyer, the Supreme Court reiterated its traditional principle that you can’t patent laws of nature, and held that the concrete steps of the patented process—administering the drug and measuring metabolite levels—were too conventional to transform an unpatentable idea into a patentable process.


The decision—and the fact that it was unanimous—is important because it reaffirms the principle that abstract ideas and laws of nature are not eligible for patent protection. But we would have liked to see Justice Breyer go further. After a series of disastrous decisions by lower courts in the 1990s, the Patent Office began granting a large number of patents, such as those for “business methods” and software, that seemed inconsistent with the Supreme Court’s ban on patenting laws of nature and abstract ideas. Those same decisions gave rise to the medical diagnostic patents that were at issue in Tuesday’s decision. But the Supreme Court has yet to address the broader question of whether the changes of the 1990s were consistent with the Supreme Court’s precedents. It ducked the question in a 2010 decision about business method patents, and Justice Breyer seems to have ducked it again in this decision. The result has been a great deal of uncertainty about what can be patented, and an explosion of patent litigation in the software industry. Mayo v. Prometheus was a step in the right direction, but it was also a missed opportunity to rule on these broader questions.