Expression of self- versus nonself-KIRs reflects a random combination of receptors. Observed frequencies of NK cells expressing nonself- (NS) and self- (S) KIRs were compared with theoretic values obtained by sequential acquisition of KIRs in the presence (Sel) or absence (No Sel) of selection by cognate ligands. (A) Frequencies of cells expressing one NS or one S KIR as their only KIR. To exclude any unintentional bias for expression of S- or NS-KIR, the analysis was restricted to individuals with 2 KIR ligands ensuring equal probabilities for both events. (B) Frequencies of NK cells expressing 2 NS, 1 NS and 1 S, or 2 S KIRs in individuals with 2 strong KIR ligands. (C) Frequencies of NK cells expressing 1 NS or 1 S KIR in individuals with 2 ligands when the KIR3DL2–HLA-A3/A11 interaction was excluded.