 Ang-1 promotes quiescence of HSC in vivo.This quiescence or slow cell cycling of HSCs induced by Tie-2/Ang-1 signaling contributes to the maintenance of long-term repopulating ability of HSC and the protection of the HSC compartment from various cellular stresses.Tie-2/Ang-1 signaling plays a critical role in the HSC that is required for the long-term maintenance and survival of HSC in bone marrow.