Tamoxifen citrate is an endocrine therapy drug used to treat breast cancer.  It acts to interrupt estrogen signaling by blocking the estrogen receptor (ER).  In vivo, tamoxifen and tamoxafin metabolites bind to estrogen receptors, causing the transcription of estrogen-response genes to be inhibited.  In vitro, Tamoxifen is known to inhibit protein kinase C through estrogen receptor-independent antineoplastic effects.  This drug therefore has important applications in breast cancer research, particularly in ER+ breast cancers.  Recent studies have shown tamoxifen-resistant cells use autophagy as a survival mechanism.