Inhibition of ErbB2 signaling may slow the development of resistance to estrogen deprivation therapy by inhibiting a pathway that promotes estrogen independent growth. The ErbB2 signal transduction pathway bypasses the requirement for estrogen for breast epithelial cell growth. When ErbB2 is activated in ER positive breast cancer cells in vitro, ER becomes phosphorylated and capable of stimulating transcription without estrogen. Chronic activation with heregulin, a ligand for the ErbB family of receptors, leads to ER down-regulation and the acquisition of an ER negative phenotype.