XBP1 and breast cancer: To confirm the crucial role of XBP1s in mediating resistance to CDK4/6 inhibitors and endocrine therapy, we assessed the responsiveness of HR+/HER2− breast cancer to the combined therapy of palbociclib and fulvestrant, with or without 4μ8C‐mediated blockade of XBP1 splicing, in PDO‐0912 and PDO‐3890 models.