To this end, KHYG-1 NK cells were cultured in the presence of IL-2 under hypoxic (1% O2) or normoxic (20% O2) conditions for 24 h and subsequently incubated with the hypoxic or normoxic tumor cell lines K562 or MM.1S at different E : T ratios for another 4 h to evaluate the cytotoxicity by flow cytometry. Here, IL2 is linked to neoplasm.