Low-level KIR3DL1 expression differs in 2 donors due to a nonsynonymous mutation at nucleotide position 115 in exon 3 encoding the D0 domain of the KIR3DL1*005 allele. NK cells from each donor were stained with anti-KIR3DL1 antibody (DX9; solid lines) or isotype control (IgG₁; dotted lines). KIR3DL1 surface expression correlated with KIR3DL1 allele typing, with donors 1 and 2 having unimodal high-level expression and donors 3 and 4 having bimodal (high- and low-level) expression. The low-level receptor expression differed between donors 3 and 4. Comparison of both KIR3DL1 sequences from donors 3 and 4 revealed a single nonsynonymous nucleotide mutation at position 115 of the KIR3DL1*005 allele in donor 3.