Notably, previous studies indicated that the MYC, known as a direct target gene of E2F1,[43] could enhance the activity of IRE1α RNase, resulting in elevated XBP1s/XBP1u ratio, and directly activate the transcription of XBP1 by binding to its promoter region.[33, 44] Hence, these results suggest that XBP1s may participate in a positive feedback loop with E2F1 and MYC in breast cancer, furthering cell proliferation and the cell cycle in breast cancer. Here, XBP1 is linked to breast cancer.