Emergence of NK-cell progenitors and NK cells in the mouse embryo. Schematic representation of where and when NK-cell progenitors and different NK-cell populations arise during fetal development. iNK indicates immature NK cells (CD3⁻NK1.1⁺DX5⁺), mNK, mature NK cells (CD3⁻NK1.1⁺DX5⁺CD43⁺Mac-1⁺), and tNK, thymic NK cells (CD3⁻CD25⁻CD122⁺NK1.1⁺Ly49D⁻IL-7R⁺). Cytotoxic NK cells were identified as CD107a⁺ using the degranulation assay.