Id1 is required for proper hematopoietic microenvironment function. (A) BMCs were harvested 4 months after Id1^−/− or Id1^+/+ BMC were transplanted into irradiated Id1^+/+ recipient mice and then analyzed for donor hematopoietic reconstitution by flow cytometry. Flow cytometric data are representative data from individual mice. The left panel shows donor myeloid (Gr-1 × Mac-1) and erythroid (TER119 × CD71) reconstitution in Id1^−/− and Id1^+/+ recipient mice. The numbers indicate the percentage of donor cells in each quadrant. The right panel shows percentages of Gr-1⁺/Mac-1⁺ or TER119⁺ BMC in all the mice analyzed (each genotype, n = 3). (B) Repopulation of Id1^+/+ BMC in irradiated Id1^−/− or Id1^+/+ recipient. Total BMC of recipient mice were enumerated 4 months after transplantation (n = 6, *P = .04). (C) BMC were harvested 4 months after Id1^+/+ BMC were transplanted into irradiated Id1^+/+ and Id1^−/− recipient mice and then analyzed for donor hematopoietic reconstitution by flow cytometry. Left panel shows myeloid and erythroid repopulation of representative mice, whereas the right panel shows percentages of Gr-1⁺/Mac-1⁺ myeloid or TER119⁺ erythroid BMC in all the mice analyzed (each genotype, n = 6, *P < .001).