X-box binding protein 1 (XBP1; also tax-responsive element-binding protein 5 (TREB5)) is a ubiquitously expressed 38kD member of the bZIP family of proteins. Human XBP1 is 261aa in length, and contains a basic motif (aa72-94), which serves as a DNA binding site, followed by a leucine-zipper domain (aa98-126). A splicing variant in isoform 1 of XBP1 produces a second isoform, which has a 210aa substitution for aa 161-261 in isoform 1. Human XBP1 shares 85%, 79%, and 77% aa sequence identity with bovine, rat, and mouse XBP1, respectively. XBP1 functions as a transcription factor that is essential for hepatocyte growth, plasma cell differentiation, immunoglobulin secretion, and the unfolded protein response.