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The asserted patents arose from a discovery that, in maternal plasma, most fetal DNA is smaller than most maternal DNA. Id. at 3-4. Following this discovery, the inventors developed "methods of preparing a fraction of cell-free DNA that is enriched in fetal DNA." Id. at 4. The majority distinguished the present case from prior patent-eligibility cases by explaining that it was "not a diagnostic case" or "a method of treatment case" but was "a method of preparation case." Id. at 8. Since Mayo, the Federal Circuit has "consistently held diagnostic claims unpatentable as directed to ineligible subject matter" and found that "method of treatment claims are patent-eligible." Id. But a method of preparation does not fit neatly into either category.

Because Illumina's claims "do not fall into either bucket" the claims merited careful consideration under the Alice/Mayo test. Slip Op. at 8. The Federal Circuit majority defined the natural phenomenon-at-issue to be "that cell-free fetal DNA tends to be shorter than cell-free maternal DNA in a mother's bloodstream." Id. at 9. It then concluded that the asserted claims were not "directed to that natural phenomenon but rather to a patent-eligible method that utilizes it." Id. Specifically, it found that the "methods include specific process steps — size discriminating and selectively removing DNA fragments that are above a specified size threshold — to increase the relative amount of fetal DNA as compared to maternal DNA in the sample." Id. The majority concluded that "[t]hose process steps change the composition of the mixture, resulting in a DNA fraction that is different form the naturally-occurring fraction in the mother's blood," meaning that "the process achieves more than simply observing that fetal DNA is shorter than maternal DNA or detecting the presence of that phenomenon." Id. at 10.