The serine/threonine-protein kinase/endoribonuclease inositol-requiring enzyme 1 (IRE1) is an enzyme that in humans is encoded by the ERN1 gene. IRE1 is an endoplasmic reticulum (ER) transmembrane sensor that activates UPR to maintain ER and cellular function. While mammalian IRE1 promotes cell survive, it can initiate apoptosis via decay of anti-apoptotic microRNAs. IRE1 activation is initiated by homotypic interactions of the stress-sensing lumenal domain favoring transautophosphorylation of the kinase-extension nuclease (KEN) domain on the cytoplasmic side of the ER membrane. IRE1/XBP-1 has been shown to regulate a variety of genes in various cell types in response to ER stress, mostly related to ER function and the secretory pathway, although the target genes vary depending on the cell type and nature of the stress stimuli.