Over the past two decades, genetically modified plants have graduated from laboratory curiosities to crops planted on millions of hectares. But while major seed firms have struggled with public worries over environmental questions and food safety, they’ve also been locked in a more private fight over a central question: who owns one of the industry’s principal gene-insertion technologies?

The dispute is coming to a head. Last year, the U.S. Patent and Trademark Office issued a broad patent to Washington University in St. Louis covering one of the industry’s most basic tools: the use of a soil bacterium to insert desired genes into the DNA of broad-leafed crops like soybean, cotton, potato and canola, conferring traits like herbicide tolerance and pest resistance. “That was the fundamental feature of our work,” says Andrew Binns, a professor at the University of Pennsylvania and one of the inventors named on the patent. “It is the basis for practically all plant genetic engineering these days.”

Agricultural biotech giant Syngenta, based in Basel, Switzerland, holds exclusive rights to the Washington University patent. Andrew Neighbour, director of the school’s technology transfer office, says the patent “certainly provides [Syngenta] with a strong position in the marketplace.” But while the granting of the patent was a watershed event-coming 17 years after its first version was filed-the issued patent is a shadow of its original self. Many of the claims originally filed are absent; these are still in dispute.

Syngenta isn’t the only seed company interested in the patent’s fate: Monsanto, Pioneer and Dow AgroSciences also make heavy use of the technique. Syngenta says it is not yet trying to collect royalties on its patent. And none of the companies would comment on the claims being fought out in the patent office, what the technology is worth, or what the financial impact might be once ownership is resolved. The patent office could rule as early as next year. But given the technology’s key role in a multibillion-dollar industry, the next stop may well be the courts.