This section contains the nature of the case and procedural background.

P filed two patents which described test results used to determine if the drug the dosage of thiopurine was high or low for a particular patient. P is the sole and exclusive licensee of the patents. It sells diagnostic tests that embody the processes the patents describe. Mayo (D) bought and used those tests. In 2004, D announced that it intended to begin using and selling its own test. P sued D for patent infringement. The District Court ultimately granted summary judgment in D's favor. The court reasoned that the patents effectively claim natural laws or natural phenomena-namely the correlations between thiopurine metabolite levels and the toxicity and efficacy of thiopurine drug dosages-and so are not patentable. The Federal Circuit reversed. It pointed out that in addition to these natural correlations, the claimed processes specify the steps of (1) 'administering a [thiopurine] drug' to a patient and (2) 'determining the [resulting metabolite] level.' These steps, it explained, involve the transformation of the human body or blood taken from the body. Thus, the patents satisfied the Circuit's 'machine or transformation test,' which the court thought sufficient to 'confine the patent monopoly within rather definite bounds.' The Supreme Court granted certiorari.

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