XBP-1 is a transcription factor containing a bZIP domain. It was first identified because of its binding ability to X-box, a conserved transcriptional element in the promoter of human HLA DR gene. XBP-1 has multiple functions. It controls MHC class II gene regulation and is also essential for differentiation of plasma cells. XBP-1 upregulates as part of the ER stress response, also known as the unfolded protein response. Unspliced XBP-1 is 261 amino acids and migrates on the SDS-PAGE around 33 kD; spliced XBP-1, recognized by clone 143F, is371 amino acids and migrates around 55 kD.