IL-15 is required for Blimp1 induction in developing NK cells. (A-B) Lineage marker–negative Sca1⁺ckit⁺ HSC-enriched progenitors were isolated from the bone marrow of Blimp1^gfp/+ or wild-type mice and cultured on OP9 cells in the presence of IL-15 (A) or on OP9-DL1 cells in the presence of IL-7 (B). GFP expression of the indicated populations was measured at day 9 of culture (filled histograms, Blimp1^gfp/+; open histograms, wild-type). Results are representative of 2 independent experiments.