XBP1 and breast cancer: Several genes that are linked to the unfolded protein response such as C/EBP homology protein (CHOP), x-box binding protein-1 (XBP-1), activating transcription factor-3 (ATF3), activating transcription factor-4 (ATF4), and eukaryotic translation initiation factor 2-alpha (eIF2α) are also highly induced in MCF7 and SUM52PE breast cancer cells when co-treated with acidosis and hypoxia [109].