Canonical Notch signaling is required for in vitro but not in vivo expansion of human HSCs. (A) dominant-negative MAML (dnMAML) blocks transcriptional activation of Notch target genes downstream of all Notch receptors. ICN, intracellular Notch; CSL, CBF1/Su(H)/Lag-1, also called RBP-Jk; CoA, coactivator. (B) dnMAML inhibits maintenance and expansion of human HSCs in coculture with OP9 stromal cells expressing Delta-like4 Notch ligands (OP9-DL4). (C) dnMAML preserves in vivo expansion of human HSCs after transplantation into xenogeneic NOD/SCID/γc^null immunodeficient mice. Professional illustration by Alice Y. Chen.