XBP1 (X-box binding protein 1) is a transcription factor belonging to the bZIP protein family. It consists of a DNA binding domain and a leucine zipper domain, primarily located in the cell nucleus. XBP1 plays a crucial role in the endoplasmic reticulum stress (ERS) response, being activated by splicing to promote the expression of adaptive molecules, such as genes related to the folding and quality control of secretory proteins, to maintain cellular homeostasis. The expression of XBP1 in different tissues and cells is specific, and mutations or dysregulation in its gene expression are associated with various diseases, including multiple myeloma, autoimmune diseases, and metabolic disorders.