A 3-stage model of NK development. The cytoplasmic CD3⁺ nuclear TdT⁺ bipotent T/NK-cell precursor may develop into either immature T cells under the actions of E2A and IL-2 or immature NK cells under the actions of ID2 and IL-15. Lineage commitment is fully established when immature NK cells express CD94 1A transcripts. The model shows the expression of the CD94 receptor on immature NK cells, but it is unclear whether it is expressed on the surface at this stage of development. Illustration by Paulette Dennis.