In normal cells, XBP1 is typically found in an inactive form known as XBP1u (unspliced). Upon activation of the UPR, due to stress such as an accumulation of misfolded proteins in the endoplasmic reticulum (ER), XBP1u undergoes unconventional splicing mediated by the enzyme IRE1. This generates the active form, XBP1s (spliced), which translocates to the nucleus to initiate the transcription of UPR target genes.
