KLRK1 and neoplasm: Each source has advantages and disadvantages; for example, PB-NK cells exhibit high expression of activating factors with the potential to destroy tumor cells (such as CD16, NKG2D, and NKp44), but collecting them from peripheral blood is expensive and time-consuming because NK cells represent only 10–15% of all lymphocytes, and subsequent methods must be used to expand them ex vivo [348,349,350].