ESR1 and breast cancer: A recent analysis of independent cohorts reported that patients with triple negative breast cancer (TNBC), a form of breast cancer in which tumor cells do not express the genes for estrogen receptor, progesterone receptor, and HER2, have a specific XBP1 gene expression signature that is strongly associated with poor prognosis; and a demonstrated a tumorgenic function for XBP1 branch of the UPR in TNBC.