Samd1 inhibits HSPC differentiation in vivo. (A) Schematic outlining the in vivo competitive transplant assay. (B) Samd1 mRNA in shRNA-expressing GFP⁺ CD45.2⁺ bone marrow at week 16 (n = 4). Normalized to 18s rRNA. (C) Quantitation of total GFP⁺ CD45.2⁺ peripheral blood (PB) at weeks 4, 8, 12, and 16 after transplant as determined by flow cytometry (n = 11). (D) Quantitation of GFP⁺ CD45.2⁺ B cells (CD19⁺), T cells (Thy1.2⁺), granulocytes (GR-1⁺ CD11b⁺), and monocytes (CD11b⁺) from PB at weeks 4, 8, 12, and 16 after transplant as determined by flow cytometry (n = 11). (E) Cell-type make-up of CD45.2⁺ fraction of PB at weeks 4, 8, 12, and 16 as determined by flow cytometry (n = 11). (F) Quantitation of CD71⁺ erythroid progenitors from 16 weeks after transplant bone marrow (CD45.2⁺ GFP⁺) as determined by flow cytometry (n = 4). (G) Quantitation of Kit MFI of CD71⁺ erythroid progenitors from 16 weeks after transplant bone marrow (CD45.2⁺ GFP⁺) as determined by flow cytometry (n = 4). (H) Quantitation of total CD45.2⁺ (GFP⁺) bone marrow at week 16 of transplant as determined by flow cytometry (n = 4). (I) Proposed model of SAMD1 mechanism in erythropoiesis. Error bars represent SEM. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001 (2-tailed unpaired Student t test). CT, control; KD, knockdown.