Natural killer activity of A91V heterozygous individuals. Nonadherent peripheral blood lymphocytes isolated from heterozygous subjects and healthy controls (PNs 01-09 and controls, respectively) were incubated for 4 hours at 37° C with ⁵¹Cr-pulsed K562 cell line. Specific lysis was measured in a triplicate assay performed with 5 × 10³ target cells mixed with effector cells at different effector-target ratios (E/T). The percentage of specific lysis was calculated as follows: 100 × (specific release – spontaneous release)/(total release – spontaneous release). Mean and SDs were calculated on 10 control individuals.