The NK Cell Expansion Kit is suitable for the ex vivo culture and expansion of human natural killer (NK) cells. It contains engineered Growth-Arrested NK Feeder Cells and NK Cell Basal Medium. The Growth-Arrested NK Feeder Cells are K562 cells engineered to express membrane bound IL-21, and other components, to drive the robust activation and expansion of NK cells. The kit provides enough growth-arrested feeder cells for a starter population of 1 million purified NK cells or PBMC cells.