In an opinion handed down on March 20, 2012, the Supreme Court determined that patents covering thiopurine dosing tests covered unpatentable subject matter. The Court held that while the application of a law of nature to a known structure or purpose may deserve patent protection, to transform an unpatentable law of nature into patent eligible technology takes more than stating the law of nature and asking that the law be applied.

The Prometheus patent claims are directed to test kits associated with the treatment of autoimmune diseases with thiopurine drugs. It had been well known that when ingested, thiopurine is broken down by the body to produce certain chemicals (or metabolites) in the bloodstream. It had also been known that there were correlations between the metabolite levels in the bloodstream and the toxicity and efficacy of thiopurine drugs. The Prometheus claims were directed to the use of those correlations of bloodstream metabolites to inform treating physicians about the level of thiopurine in their patients and how those levels needed to be adjusted for maximum benefit.

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