KIR acquisition is independent of cognate HLA ligands. The observed frequencies of NK cells were compared with those predicted by the sequential selection model. (A) Frequencies of NK cells expressing zero to 4 KIRs in individuals with 1 (n = 9), 2 (n = 20), 3 (n = 9), and 4 (n = 3) KIR ligands are shown. Importantly, nonfunctional KIR ligands including HLA-B13 and HLA-A25 were considered nonself-ligands.^18,28  Boxes represent the median and the 5th and 95th percentile. Red bars indicate values obtained by modeling KIR expression frequencies selected by 1 to 4 self-KIR ligands. (B) Frequencies of NK cells expressing zero to 4 KIRs in individuals with intermediate-KIR repertoires (n = 9) and variable number of strong KIR ligands were compared with those generated under selection by 1, 2, 3, and 4 ligands, respectively.