THE nine justices on America's Supreme Court must apply their minds to the thorniest of legal questions. On April 15th they were presented with a particularly testing puzzler: should the law allow people to patent human genes? The case is the culmination of a battle that began in 2009. It pits America's Association for Molecular Pathology and various other interested parties - represented by lawyers from the American Civil Liberties Union - against Myriad Genetics, a biotechnology firm that holds patents on two human genes, called BRCA1 and BRCA2. That makes Myriad the sole producer of tests to detect mutations in those genes. Such mutations often increase a woman's risk of developing breast or ovarian cancer. It is an emotionally charged question. Patent offices around the world have been granting patents on genes, human and otherwise, for decades, but time has done little to soften opposition to the idea. Laymen are often baffled when they find out that it is possible to patent parts of the human genome. Anti-patenting campaigners argue that the idea of claiming a patent over the shared genetic heritage of the human race is absurd, immoral or both. But the court is unlikely to base its ruling on such airy ethical principles. The legal debate revolves not around the two genes" humanness, but their naturalness - for the judges are being asked to rule whether Myriad's genes count as a "product of nature." If they do, then under American law they cannot be patented. At first glance, it may seem odd to argue that a gene is anything other than a product of the natural world. But Myriad's patents do not cover the genes as they occur in living cells. Rather, they cover isolated forms of the genes, which have been identified, snipped from the genome and chemically modified to make them analysable in a laboratory. Myriad argues that these alterations, and the new use to which the resulting gene is put as part of a diagnostic test, make their product sufficiently innovative to be worthy of the law's protection. The plaintiffs (and the judge who first ruled against Myriad in 2010) say this reasoning is spurious: the chemical changes are trivial, and the resulting bit of DNA useful only because it is identical in every meaningful way to the version that occurs in vivo. Doctors, patients and biotechnology companies are interested in the information content of genes, the argument runs, not the incidental details of their chemical composition. For a while, the court found itself bogged down in analogies with the patent-eligibility of chocolate-chip cookies and baseball bats. There are other criticisms of Myriad. One is that its patents are too broad. The firm's tests look for variations in BRCA genes. Rather than patenting every conceivable variant, the company claims jurisdiction over isolated DNA sequences, 15 base pairs long, that match short parts of the genes. The bases in question are the chemical "letters" of the genetic code. An analysis by Jeffrey Rosenfeld and Christopher Mason, of the Sackler Institute for Comparative Genomics and Cornell University respectively, published in Genome Medicine, found that 689 other human genes contain 15-base-pair strings found in BRCA1, suggesting Myriad's patents technically cover those unrelated genes, too. The economics of genomics The arguments ranged beyond the arcana of human genetics and American patent law. The judges were keen to explore the impact of a hypothetical anti-Myriad ruling on the country's thriving biotechnology industry, which, according to one estimate, was worth $92 billion in 2011. Several questioned Christopher Hansen, the plaintiffs" lawyer, on what incentive firms would have to identify interesting genes if there were no possibility of winning a patent on them. Developing such tests is not cheap. According to Mark Capone, Myriad's president, the firm spent around $500m before it had a product it could bring to market. Others argue that such patents hold back biotechnology, an industry that is changing fast. Donald Verrilli, America's solicitor general, who represents the government's view in the case (it supports neither side), pointed out that competitors need the patent-holder's permission to isolate the genes to see if they can develop a better test, or even to examine them for any other effects they may have. Some observers think an anti-Myriad ruling would have little practical effect. The firm's patents expire in 2015, and technology has moved on. The cost of genetic sequencing is falling fast, and it may soon be possible for people to have their entire genomes sequenced for less than the roughly $4,000 Myriad charges for its tests. Any genes of interest could thus be examined for worrisome mutations, probably without infringing any patents. And Mr Hansen conceded that much work in modern biotech concerns recombinant DNA, which involves cutting and pasting DNA from different sources to make synthetic sequences not found in nature, something that the plaintiffs agree should be covered by patent law. Whatever conclusion the court comes to, the judges seemed keen to avoid an overly broad ruling. That may be a legally sensible approach to a knotty question combining genetics, economics and ethics. But, like all good compromises, it risks leaving no one satisfied. Ideally, America's Congress would set out exactly what parts of the human genome should be patentable. There is not much chance of that actually happening, though.