The use of feeder cells, such as EBV-LCL which naturally express the ligand for 4-1BB, a stimulatory receptor on NK cells, in combination with the cytokines IL-21 and IL-2, has been shown to be a highly efficient approach for ex vivo expansion of human NK cells with high anti-tumour activity [21, 43]. The gene discussed is IL21; the disease is neoplasm.