Impaired NK cytotoxicity. (A) Pooled data from 4 independent NK cytotoxicity assays from the patient (•) and controls (▵) including both parents and 2 patients with HPS of unknown genetic cause (▴). K562 cells were used as targets; spontaneous lysis was less than 18% in all assays. (B) NK cytotoxicity using PBMCs ex vivo or after 7-day stimulation with IL-2 from the patient and mean plus or minus standard deviation of 5 controls, including both parents.