ERBB2 and breast cancer: The relatively low toxicity of PRL antagonists and PRLR neutralizing antibodies and widespread PRLR expression across different breast cancer subtypes have prompted their development as delivery vehicles for other therapeutic agents, including cytotoxic compounds (121, 135, 150, 151), anti-HER2 (152), and immunomodulators to attract and/or activate CD8+ T cells (135, 153).