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Trail⁺DX5⁻Eomes⁻ natural killer (NK) cells arise in the mouse fetal liver and persist in the adult liver. Their relationships with Trail⁻DX5⁺ NK cells remain controversial. We generated a novel Eomes-GFP reporter murine model to address this question. We found that Eomes⁻ NK cells are not precursors of classical Eomes⁺ NK cells but rather constitute a distinct lineage of innate lymphoid cells. Eomes⁻ NK cells are strictly dependent on both T-bet and IL-15, similarly to NKT cells. We observed that, in the liver, expression of T-bet in progenitors represses Eomes expression and the development of Eomes⁺ NK cells. Reciprocally, the bone marrow (BM) microenvironment restricts T-bet expression in developing NK cells. Ectopic expression of T-bet forces the development of Eomes⁻ NK cells, demonstrating that repression of T-bet is essential for the development of Eomes⁺ NK cells. Gene profile analyses show that …