X-box binding protein 1, also known as XBP1, is a protein which in humans is encoded by the XBP1 gene. The XBP1 gene is located on chromosome 22 while a closely related pseudogene has been identified and localized to chromosome 5. 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. Discovery The X-box binding protein 1 (XBP1) is a transcription factor containing a bZIP domain. 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. Function MHC class II gene regulation The expression of this protein is required for the transcription of a subset of class II major histocompatibility genes. Furthermore, XBP1 heterodimerizes with other bZIP transcription factors such as c-fos. XBP1 expression is controlled by the cytokine IL-4 and the antibody IGHM. XBP1 in turn controls the expression of IL-6 which promotes plasma cell growth and of immunoglobulins in B lymphocytes. Plasma cell differentiation XBP1 is also essential for differentiation of plasma cells (a type of antibody secreting immune cell). This differentiation requires not only the expression of XBP1 but the expression of the spliced isoform of XBP1s. XBP1 regulates plasma cell differentiation independent of its known functions in the endoplasmic reticulum stress response (see below). Without normal expression of XBP1, two important plasma cell differentiation-related genes, IRF4 and Blimp1, are misregulated, and XBP1-lacking plasma cells fail to colonize their long-lived niches in the bone marrow and to sustain antibody secretion. Eosinophil differentiation XBP1 is required for eosinophil differentiation. Eosinophils lacking XBP1 exhibit defects in granule proteins. Angiogenesis XBP1 acts to regulate endothelial cell proliferation through growth factor pathways, leading to angiogenesis. Additionally, XBP1 protects endothelial cells from oxidative stress by interacting with HDAC3. Viral replication This protein has also been identified as a cellular transcription factor that binds to an enhancer in the promoter of the Human T-lymphotropic virus 1. The generation of XBP1s during plasma cell differentiation also seems to be the cue for Kaposi's sarcoma-associated herpesvirus and Epstein Barr virus reactivation from latency. Endoplasmic reticulum stress response XBP1 is part of the endoplasmic reticulum (ER) stress response, the unfolded protein response (UPR). Conditions that exceed capacity of the ER provoke ER stress and trigger the unfolded protein response (UPR). As