In 2004, it was found that the killer immunoglobulin receptor (KIR) on natural killer (NK) cells in the maternal decidua recognizes the polymorphic histocompatibility antigen HLA-C on the fetal trophectoderm as a key factor in the development of preeclampsia, thus proposing that patients with KIR A haploid (KIR AA) are at increased risk of developing preeclampsia. The gene discussed is HLA-C; the disease is preeclampsia.