PDCD1 and cancer: Similarly to what was shown for NK cells in murine cancer models [62], this study on human NK cells also confirmed that PD-1 expression is not associated to exhaustion but rather to cell activation: PD-1+ and PD-1− NK cells could comparably respond to cytokine stimulation and triggering of activating receptors, while only direct PD-1 engagement had an inhibitory effect [72].