A therapeutic method for inhibiting tumor development or growth, comprising administering an agent capable of inhibiting human delta-like ligand 4 (DII4) activity to a subject in need thereof. In one embodiment, the agent is an anti-DII4 antibody or antibody fragment capable of inhibiting the binding of DII4 to a Notch receptor. In another embodiment, the agent is a fusion protein comprising the extracellular domain of DII4 or a fragment or variant thereof, fused to a multimerizing component such as an Fc domain. The method of the invention is useful for inhibiting tumor growth, particularly in tumors which are not responsive to other therapeutic agents.
