FLT3 receptor is dispensable for reconstitution and expansion of adult HSCs after BM transplantation. Lethally irradiated adult WT recipient (CD45.1) mice were transplanted with 10⁶ unfractionated Flk2^−/− (CD45.2) or WT (CD45.2) BM cells together with 10⁶ unfractionated WT CD45.1 competitor BM cells. (A) Donor-derived PB total (Tot), B-cell, T-cell, and myeloid (M) reconstitution at 16 weeks after transplantation. Mean (SEM) values from 12 to 14 primary recipient mice of each genotype, from 2 independent experiments. At 16 weeks after transplantation recipient mice were killed, and BM cells were counted and analyzed for donor-derived myeloid (Mac-1⁺) reconstitution (B) as well as donor-derived regeneration of LSK and LSKCD150⁺ compartments (C). Mean (SEM) values for proportion of donor cells of 12 to 14 mice per genotype, from 2 independent experiments. (D) Test cell–derived PB total, B-, T-, and M-cell reconstitution 24 to 28 weeks after transplantation in secondary WT recipients. Data are expressed as mean (SEM) percentage of test cell reconstitution (9-10 recipient mice per genotype, from 2 independent experiments). ns indicates not significant; **P < .01, ***P < .001.