Mx1⁺ stromal cells control engraftment of transplanted HSPCs. (A) Total PB reconstitution of nonconditioned control recipient mice (infused with 10⁶ and 8 × 10⁶ CD45.1 congenic BM cells) and mutant recipient mice (infused with 10⁶, 4 × 10⁶, and 8 × 10⁶ CD45.1 congenic BM cells). (B) PB analysis 16 weeks after nonconditioned transplantation of 8 × 10⁶ CD45.1 congenic BM cells into Ext1 mutant recipient mice showing the contribution to B cells (B220), myeloid cells (Mac1 and Gr1), and T cells (CD3). (C) Proposed model for the functional role of Ext1/HSPG in the niche. Data are representative of 2 independent experiments; n = 5 to 8 mice per genotype. Data are represented as mean ± SD. *P < .05; **P < .01.