Background
This gene encodes a transcription factor that regulates MHC class II genes by binding to a
promoter element referred to as an X box. This gene product is a bZIP protein, which was
also identified as a cellular transcription factor that binds to an enhancer in the promoter of
the T cell leukemia virus type 1 promoter. It may increase expression of viral proteins by
acting as the DNA binding partner of a viral transactivator. It has been found that upon
accumulation of unfolded proteins in the endoplasmic reticulum (ER), the mRNA of this gene
is processed to an active form by an unconventional splicing mechanism that is mediated by
the endonuclease inositol-requiring enzyme 1 (IRE1). The resulting loss of 26 nt from the
spliced mRNA causes a frame-shift and an isoform XBP1(S), which is the functionally active
transcription factor. The isoform encoded by the unspliced mRNA, XBP1(U), is constitutively
expressed, and thought to function as a negative feedback regulator of XBP1(S), which shuts
off transcription of target genes during the recovery phase of ER stress. A pseudogene of
XBP1 has been identified and localized to chromosome 5
