Mayo Collaborative Services v. Prometheus Laboratories, Inc.

Prometheus Laboratories (plaintiff) patented methods for measuring thiopurine metabolite levels to determine whether a drug dosage for treating conditions like leukemia was too high or too low, and sold diagnostic tests embodying those methods. Mayo Collaborative Services (defendant) initially bought Prometheus's tests but later announced it would sell its own test using a different toxicity threshold, prompting Prometheus to sue for patent infringement. The district court granted Mayo summary judgment, finding the patents claimed unpatentable natural phenomena, but the Federal Circuit reversed and upheld the patents.