Natural Killer (NK) cells are a distinct sub-population of lymphocytes that have the capacity to lyse tumor cells. Human NK cells are characterized by the expression of CD56, the absence of CD3, and the secretion of cytokines, including IFN-gamma, TNF-beta, TNF-alpha, GM-CSF, IL-10, and IL-13. Cytokine-induced killer (CIK) cells exhibit phenotypic and functional similarities to both T cells and NK cells. While CIK cells express CD3 and expand readily in culture like T cells, they do not require functional priming for in vivo activity, a feature shared with NK cells. CD3+CD56+ CIK cells have potent cytotoxic functions and express a number of cytokines, including IL-2, IFN-gamma, TNF-alpha, and GM-CSF. The ability to expand CIK or NK cell populations ex vivo is valuable to researchers who require these cells for downstream applications including studies for cell therapy and cancer immunotherapy.