X-box binding protein 1, also known as XBP1, is a protein which in humans is encoded by the XBP1 gene. The XBP1 protein is a transcription factor that regulates the expression of genes important to the proper functioning of the immune system and in the cellular stress response. It was first identified by its ability to bind to the Xbox, a conserved transcriptional element in the promoter of the human leukocyte antigen (HLA) DR alpha. Abnormalities in XBP1 lead to a heightened ER stress and subsequently causes a heightened susceptibility for inflammatory processes that may contribute to Alzheimer's disease. In the colon, XBP1 anomalies have been linked to Crohn's disease.