Described herein is a previously unknown function of XBP1 in triple-negative breast cancer (TNBC). It is shown that XBP1 is preferentially spliced and activated in TNBC, and that deletion of XBP1 significantly blocks triple negative breast tumor growth. Strikingly, XBP1 is required for the self-renewal of breast tumor initiating cells (TICs). Genome-wide mapping of the XBP1 transcriptional regulatory network identified a fundamental role for XBP1 in regulating the response to hypoxia via the transcription factor hypoxia-inducible factor 1α (HIF1α). Importantly, activation of this pathway appears to carry prognostic implications, as expression of the XBP1-dependent signature is associated with shorter survival times in human TNBC.
