- The dominant lymphocytes in healthy human and murine implantation sites are pregnancy-associated uterine natural killer (uNK) cells. These cells produce 90% of pregnancy-induced, uterine interferon (IFN)- gamma, a cytokine that regulates expression of more than 0.5% of the mouse genome. Implantation sites in uNK cell-deficient and IFN- gamma -signal-disrupted mice display anomalies in decidua and its spiral arteries. Reconstitution of uNK cell-deficient females with bone marrow containing normal NK cell progenitors, establishes uNK cells and reverses the anomalies. Grafts from IFN- gamma(-/-)mice are restored uNK cells, but the uNK cells did not reverse the phenotypes. This review focuses on the functions of uNK cell-derived IFN- gamma and the genes that it may regulate in the pregnant uterus.