NOTCH1 and B-cell chronic lymphocytic leukemia: After 24 h, NOTCH1-mutated CLL cells cocultured with OP9-DLL4 showed the strongest activation of Notch1 in comparison with the other ligands (Fig. 1b), suggesting that DLL4 could be the main ligand responsible for Notch activation in CLL both under soluble and immobilized conditions.