The present invention relates to compositions and methods for characterizing, diagnosing, and treating cancer. In particular the invention provides the means and methods for the diagnosis, characterization, prognosis and treatment of cancer and specifically targeting cancer stem cells. The present invention provides an antibody that specifically binds to a non-ligand binding region of the extracellular domain of a human NOTCH receptor and inhibits growth of tumor cells. The present invention further provides a method of treating cancer, the method comprising administering a therapeutically effective amount of an antibody that specifically binds to a non-ligand binding region of the extracellular domain of a human NOTCH receptor protein and inhibits growth of tumor cells.