Despite normal numbers of LT-HSCs, Ott1-deleted BM cannot compete with WT BM during engraftment or maintenance. (A) Absolute cell numbers in BM determined by flow cytometry of LSK-gated populations stained with either CD150/CD48 or CD34/Flt3 (Flk2) antibodies. BM was harvested from Ott1^flox/null Mx1-cre (KO) or littermates possessing a WT Ott1 allele (control) 6 weeks after pIpC injection. (B) Competitive repopulation of lethally irradiated homozygous CD45.1 recipients transplanted with donor marrow from either Ott1^flox/null Mx1-cre (Ott1 KO) or littermates possessing a WT Ott1 allele (control) 6 weeks after pIpC injection. Donor marrow was injected in 4:1 excess with WT CD45.1/45.2 helper marrow. y-axis represents percentage of graft arising from the donor (CD45.2 homozygous) in peripheral blood granulocytes (PB) or BM during the stated posttransplantation interval (n = 4). Flow panels (above) are representative CD45.2-gated PB samples at 16 weeks after transplantation. (C) Maintenance after transplantation. Unexcised BM from Ott1^flox/null Mx1-cre (KO) or littermates possessing a WT Ott1 allele (control) was engrafted into lethally irradiated CD45.1 recipients 1:1 with CD45.1/CD45.2 competitor marrow and then excised with pIpC. Percentage is homozygous CD45.2 donor in peripheral blood leukocytes at the stated interval after pIpC (n = 5). Representative flow panels show CD45.2 donor percentage (bottom right quadrant) in PB before and after pIpC. Ctl, top panels; KO, bottom panels. Graphs represent mean values, and error bars represent SD.