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XBP1 is also named as XBP2, belongs to the bZIP family. The X-box-binding protein-1 (XBP1) is a transcriptional regulator of the ER stress response that lies downstream of inositol-requiring enzyme 1 (IRE1α) activation(PMID: 14559994). IRE1α possesses both kinase and ribonuclease activity and processes XBP1 mRNA to produce an active transcription factor in response to ER stress (PMID: 11779464, 11780124).It has been found that upon accumulation of unfolded proteins in the endoplasmic reticulum , the mRNA of this gene is processed to an active form by an unconventional splicing mechanism that is mediated by the endonuclease inositol-requiring enzyme 1.The resulting loss of 26 nt from the spliced mRNA causes a frame-shift and an isoform XBP1, which is the functionally active transcription factor. The isoform encoded by the unspliced mRNA, XBP1(U), is constitutively expressed, and thought to function as a negative feedback regulator of XBP1(S), which shuts off transcription of target genes during the recovery phase of ER stress(11850408). The XBP1 antibody (24168-1-AP) can detect both isofroms XBP1(U) and XBP1(S).