CD9 and cancer: Consistent with a recent study that indicated that exposure to cancer cells causes NK cells to lose their cytotoxic ability (Chan et al., 2020), we identified the tendency of NK cells to differentiate to an resting state with decreasing expression of activated marker KIR2DL4 as well as increasing expression of decidual-like NK cells markers: CD9, CD49a, and PD-1 (Albini and Noonan, 2021), which are only triggered by cell-to-cell interactions as validated by FACS.