Angptl3 is required for the quiescence of HSCs in vivo. (A) Relative frequency of LT-HSCs as Lin⁻Sca-1⁺Kit⁺Flk2⁻CD34⁻ cells in WT and Angptl3-null BM at 8-12 weeks (*P < .05, n = 16). (B) The major hematopoietic lineages were normal in Angptl3-null mice. The major lineages of hematopoietic cells in BM in WT and Angptl3-null mice were quantitated by flow cytometry analysis as T cells (CD3), B cells (B220), myeloid cells (Mac-1 and Gr-1), and erythroid cells (CD71⁻Ter119⁺, CD71⁺Ter119⁺, and CD71⁺Ter119⁻). (C) Angptl3-null BM HSCs are less quiescent than WT HSCs. In the left panel, LT-HSCs as Lin⁻Sca-1⁺Kit⁺Flk2⁻CD34⁻ cells from a representative WT or Angptl3-null mouse, stained with Hoechst 33 342 and pyronin Y, were analyzed for cell cycle stage. In the right panel, the percentages of G₀ cells for Angptl3-null and WT cells are shown (*P < .05, n = 9, right). (D) BrdU incorporation indicates decreased cycling in HSCs isolated from WT mice compared with those isolated from Angptl3-null mice (*P < .05, n = 5). (E) Angptl3-null mice were more sensitive than WT mice to myelotoxic injury. WT or null mice were treated with 150 mg/kg 5-FU intraperitoneally as described in Methods. Survival of the 2 groups was analyzed using a log-rank nonparametric test (P < .05, n = 10-14 in each group) and shown as a Kaplan-Meier survival plot.