Coexpression of multiple KIRs is more frequent than expected by the product rule. (A) The observed frequencies of NK cells coexpressing the indicated KIRs in 44 haplotype A donors are plotted against those given by the product of the expression frequencies of the respective KIRs. Combinations including KIR3DL1 were based on 40 individuals, since 4 donors had KIR3DL1 alleles, which were not expressed at the cell surface.²⁶  A perfect fit to the product rule is shown as a gray line, representing a 1:1 relationship between observed and expected frequencies. (B) The observed frequencies of NK cells coexpressing 2, 3, and 4 inhibitory KIRs are plotted against frequencies expected by the product rule. Pooled data from distinct combinations of 2 (left panel), 3 (middle panel), and 4 (right panel) KIRs are shown.