CD57 is acquired after activation of CD57⁻CD56^dim NK cells. Sorted CD57⁻CD56^dim NK cells were cultured with 500 IU/mL or 1000 IU/mL IL-2 for 5 days. Representative histograms of CD57 expression by the NK cells before sort, after sort, and after stimulation are shown (shaded area represents isotype-matched control Ig; and bold line, CD57 stained cells).