This case challenges the validity of several patents that address the correlation between a class of drugs and the chemical changes they produce in the body. The patent holder, Prometheus Laboratories, does not claim a patent on the drug or on the blood test used to measure metabolite levels after the drug has been administered. Prometheus does claim a patent on the correlation between the two and the fact that metabolite levels may indicate a need to adjust the drug dosage. In its amicus brief, the ACLU argues that this correlation is not patentable under the Patent Act or the First Amendment because it represents an abstract idea. We also argue that the First Amendment would prohibit the patent claim even if the statute allowed it because the patent attempts to license pure thought.