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The Supreme Court’s decision last Term in Mayo v. Prometheus left considerable uncertainty as to the boundaries of patentable subject matter for molecular diagnostic inventions. First, the Court took an expansive approach to what counts as an unpatentable natural law by applying that term to the relationship set forth in the challenged patent between a patient’s levels of a drug metabolite and the indication of a need to adjust the patient’s drug dosage. And second, in evaluating whether the patent claims add enough to this unpatentable natural law to be patent eligible, the Court did not consult precedents concerning the patentability of claims involving natural laws and natural products. Instead, it turned to two seemingly inconsistent decisions that reached opposing conclusions concerning the patent eligibility of industrial methods that used mathematical algorithms. The Court’s analysis invites challenges to many issued patents, while offering little guidance for resolving them. This Term, in the Association for Molecular Pathology case, the Court has another opportunity to clarify the meaning of its exclusion of natural phenomena from patent eligibility.