CD56^dim NKG2A⁻ KIR⁻ cells are hyporesponsive to HLA class I–deficient target cells. Enriched NK cells were incubated alone, or after 4 hours of exposure to class I–deficient K562 or 221 cells. Cells were then stained for CD56, CD3, NKG2A, and KIR, and the degranulation antigen CD107a. Shown is a representative example gated on receptor-negative or -positive cells based on 6 experiments showing a similar pattern. The percents listed (M1 gate) represent positive events based on an isotype-matched control antibody.