DLL4 and neoplasm: Accumulating evidence has demonstrated that the use of a single anti-angiogenic agent, the strategy most often used now, is unable to sufficiently inhibit tumor angiogenesis, leading to tumor recurrence and drug resistance; as tumor endothelial cells can still be stimulated by alternative angiogenic factors, like angiopoietin-2 (Ang-2) and Dll4, which are not blocked [92].