XBP1 and breast cancer: It can promote the invasion and proliferation of BC cells by regulating the XBP1-HIF-1α pathway and HER-2 pathway in MDA-MB-23 and MDA-MD-435 cell lines, respectively [194], and assist in promoting trastuzumab resistance in HER2 overexpressing BC cells via FOXO1 and PI3/Akt pathway [195].