Use of prolactin receptor antagonists in combination with an agent that inactivates the HER2/neu signaling pathway

1. A method for inhibiting a breast cancer cell proliferation, inducing apoptosis in a breast cancer cell, or breast slowing tumor growth consisting of exposing a target breast cancer cell to a prolactin receptor antagonist and an agent that inactivates the HER2/neu signaling pathway, wherein the prolactin receptor antagonist is a human prolactin in which a glycine residue at position 129 is substituted with an arginine;

wherein said prolactin receptor antagonist interferes with the prolactin signaling pathway;

wherein said agent that inactivates the HER2/neu signaling pathway is an antibody against a HER2;

and wherein said target breast cancer cell expresses a prolactin receptor.

2. The method of claim 1 , wherein the agent that inactivates the HER2/neu signaling pathway is trastuzamab.

3. The method of claim 2 , wherein trastuzamab is administered concurrently or sequentially with the prolactin receptor antagonist.