Thiopurine is a drug that has been used to treat various illnesses, such as acute leukemia and autoimmune disorders. When a person ingests a thiopurine compound, the body metabolizes it in a manner that frequently differs from most other individuals. As a result, it was difficult to discern whether a particular dosage given to an individual was too high, risking harmful side effects, or too low and thus ineffective. Prometheus Laboratories, Inc. (Prometheus) (plaintiff) filed two patent applications that described the findings of tests used to determine whether a particular thiopurine dosage was too high or too low. Thereafter, Prometheus created diagnostic tests for sale that embodied the processes the patents described. Mayo Collaborative Services and Mayo Clinic Rochester (collectively Mayo) (defendant) purchased the diagnostic tests created by Prometheus. Several years later, Mayo announced that it intended to sell its own test, using a different thiopurine level to determine toxicity. Prometheus filed suit against Mayo in federal district court claiming patent infringement. The district court granted Mayo’s motion for summary judgment. In doing so, the court noted that Prometheus’ patents claimed natural phenomena that could not be patented. The court of appeals for the federal circuit reversed and upheld the patent applications. The U.S. Supreme Court granted certiorari to review.