DLL1 and acute lymphoblastic leukemia: Signals initiated by growth factors, such as interleukin 7 (IL-7; Barata et al, 2001) or insulin growth factor 1 (IGF-1; Medyouf et al, 2011), or by receptor–ligand interactions, such as NOTCH-Delta-like1/4 (DL1/4; Weng et al, 2003; Armstrong et al, 2009), are crucial for T-ALL development.