XBP1 and breast cancer: Overexpression of XBP1 in MCF7 and LCC1 anti-estrogen-sensitive breast cancer cells resulted primarily in an increase in XBP1s and an induction of RelA expression at both the mRNA and protein levels [22] Chen et.al reported that XBP1 was activated in triple negative breast cancer, a highly aggressive malignant subtype of breast cancer.