What is the difference between having a negative or a positive receptor in cancer?

A receptor is. The equivalent of a lock; and its 'ligand' the key...It may start cell cycle. In breast cancer her-2-neu can be "over-expressed" as a receptor on cell surface, so the ligand can start cell growth easily and the trastuzumab blocks the ligand's access. Estrogen and Progesterone are similar in action, differnt ligands. There are other examples in other tumors.