XBP1 and triple-negative breast carcinoma: The active spliced isoform of X-box binding protein 1 (XBP-1), a downstream effector of IRE1α known to regulate the tumorigenicity and progression of triple-negative breast cancer, is enriched in tumor-initiating cells and cooperates in the transactivation of known hypoxia-inducible factor 1 (HIF1) targets49.