Comparison of experimental data with different models of KIR acquisition and identification of 3 distinct KIR repertoires. (A-C) Observed frequencies of NK cells expressing zero to 4 KIRs were compared with those predicted by sequential acquisition of KIRs under different conditions (red bars). (A) Binomial distribution with equal and constant probabilities (P = .25) of KIR acquisition. (B) Binomial distribution with individual probabilities for acquisition of different KIRs in each individual. (C) Beta-binomial distribution describing the best possible fit to the observed data and corresponding to increasing KIR acquisition probabilities depending on the number of KIRs expressed at the cell surface. (D-F) Identification of 3 distinct KIR repertoires characterized by (D) low-KIR, n = 9; (E) intermediate-KIR, n = 9; and (F) high-KIR, n = 11, expression. Low-KIR repertoires were defined as more than 40% KIR-negative NK cells and high-KIR repertoires were defined as less than 20% KIR-negative NK cells.