DLL1 and B-cell chronic lymphocytic leukemia: To further confirm the potent stimulation of Notch signaling by Delta-like ligands in an immobilized model, we cocultured primary CLL cells with the stromal cells OP9 expressing the different human Notch ligands Jagged1 (OP9-JAG1), DLL1 (OP9-DLL1), and DLL4 (OP9-DLL4).