PATENTS are supposed to encourage innovation, not stifle it. On March 20th America's Supreme Court threw out two medical patents for doing the latter. The ruling in Mayo v Prometheus was unequivocal. So was the horrified reaction from the biotechnology industry. Prometheus is the most important case to date for biotech's most important new effort. "Personalised medicine" promises new treatments and much-needed new revenue. Different patients are predisposed to certain diseases and certain remedies. If firms understand these predispositions, they can offer diagnostic tests and targeted treatments. Not surprisingly, companies have sought patents for many tests. The Supreme Court may have made some of these patents worthless. Prometheus, a subsidiary of Nestlé, had patented a test to determine the correct dose of thiopurines, drugs that have long been used to treat gastrointestinal disorders. Thiopurines" effect depends on how each patient processes the drug. Prometheus patented a way to determine the best dose for a given patient: concentrations of certain chemicals in the blood should be within a range, high enough to work but low enough to be safe. It sells its test to hospitals, including the prestigious Mayo Clinic. In 2004 Mayo developed a competing test with a different recommended range. Prometheus sued. In the subsequent eight years of litigation, arguments centred on a basic proposition. American law prohibits patents of nature. Mayo said that Prometheus had claimed ownership of a natural process. Big doctors" and hospitals" lobbies agreed. Uphold Prometheus's patents, they argued, and companies would rush to patent natural phenomena. The inevitable legal minefield would stifle new discoveries, they insisted. The Biotechnology Industry Organisation (BIO), a lobby, retorted that patents have long covered clever applications of natural laws. For example, a genetic mutation can identify patients who are susceptible to a given disease or treatment. The mutation is a natural occurrence, as is the reaction to the drug. But the invention comes in connecting the dots between these elements. Mayo itself, BIO pointed out, has licensed a test for a genetic mutation that predicts side-effects for a certain colon-cancer drug. If the court overturned Prometheus's patents, hundreds of others would capsize too, the industry warned. Yet the Supreme Court sided unanimously with Mayo. Stephen Breyer, writing the court's opinion, affirmed that Prometheus's patents claimed a natural law and would restrict further innovation. Administering thiopurines, observing the body's reaction and offering dosing advice did not add up to a patentable process. "Einstein could not patent his celebrated law that E=mc2," wrote Mr Breyer. Nor could Einstein have patented the observation by "simply telling linear accelerator operators to refer to the law to determine how much energy an amount of mass has produced." The biotechnology industry did not expect the ruling. It is now in a minor panic. Personalised medicine inevitably includes the application of natural laws. It is unclear which applications may be patented. The patent office and lower courts must now try to make sense of the ruling. BIO's annual conference usually features a crowded session on patent law. This year's meeting may need a bigger room.