Notably, 4/5 subjects who developed GvHD received donor NK cell with the KIR inhibitory A/A haplotype (inhibitory KIRs with either KIR2DS4 or no activating KIRs), whereas 3 of the 4 subjects without GvHD had donors of the activating B/x haplotype (having any activating KIR other than KIR2DS4) (P = 0.1 by χ-square, 2-tailed). Here, KIR2DS4 is linked to graft versus host disease.