Sequential acquisition of KIR during NK cell differentiation. In vitro differentiation of NK cells from hematopoietic stem cells was performed as described in “Methods.” RT-PCR results from a donor with KIR genotype 5 (KIR2DL2⁺KIR2DL3⁻; A) and a donor with KIR genotype 1 (KIR2DL2⁻KIR2DL3⁺; B). Neural cell adhesion molecule (NCAM, CD56) intensity was used as correlate for NK cell development, β-actin as positive control for PCR. (C) Expression levels of KIR2DL1, KIR2DL2, KIR2DL3, and NCAM normalized against β-actin from a donor with KIR genotype 7 (KIR2DL2⁺KIR2DL3⁺) were measured with real-time PCR. NCAM levels greater than 600 are indicated.