Gabbr1^−/−mutant HSCs have reduced engraftment ability in competitive transplants. (A-C) Primary competitive BM transplantation (BMT). (A) Three hundred fifty HSCs from WT and Gabbr1^−/− mice were sorted and transplanted into irradiated CD45.1 recipients, together with 350 HSCs from CD45.1/2 mice. (B) Mice were analyzed for reconstitution of PB with donor-derived CD45.2 cells at 1, 2, and 3 months posttransplant. (C) Three months posttransplant, BM CD45.2⁺ LSK progenitors and HSCs were still reduced (n = 5 recipients per group). (D-E) Secondary transplantation. Three hundred fifty WT or Gabbr1^−/− mutant HSCs from primary recipient mice were sorted and transplanted into irradiated CD45.1 recipients, together with 350 sorted HSCs from CD45.1/2 mice. (D) Mice were analyzed for reconstitution of PB with donor-derived CD45.2 cells at 1, 2, and 3 months posttransplant for donor reconstitution. (E) Three months postsecondary transplant, BM CD45.2⁺ LSK progenitors and HSCs were still reduced (n = 5 recipients per group). Data are presented as mean plus or minus SD. (WT vs Gabbr1^−/− mice: *P < .05; **P < .01; ***P < .001) (see also supplemental Figure 5).