What am I missing??????????

How can Sandisk sue KingstonTechnology for patent infringement when Sandisk does not own the original patent??

"In this case, SanDisk sued Kingston Technology Co., Inc. and Kingston Technology Group (collectively, “Kingston”) for infringing several SanDisk patents by making, using, importing, and selling certain flash memory devices such as USB thumb drives. Following the district court’s claim-construction order, the court entered judgment in favor of Kingston."