Celsis In Vitro, Inc. (Celsis) (plaintiff) patented a method of preserving liver cells, or hepatocytes, through freezing the cells cryogenically several times. This patented technique made more hepatocytes available to research scientists. The method also made it possible to provide scientists with samples consisting of a pool of hepatocytes from multiple donors who had died at different times. Celsis sold a product practicing its patent, LiverPool, to researchers. CellzDirect, Inc. (defendant) and Invitrogen Corporation, which later became Life Technologies Corporation (LTC) (defendant), also sold pooled multi-cryopreserved hepatocyte products. Celsis sued LTC for patent infringement, alleging that LTC created its products using the same method that was patented by Celsis. The district court granted Celsis a preliminary injunction against LTC. LTC appealed.