[7] 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.

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.

[6] The expression of this protein is required for the transcription of a subset of class II major histocompatibility genes.

[14] 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.

[15] 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.

Activated IRE1 catalyses the excision of a 26 nucleotide unconventional intron from ubiquitously expressed XBP1u mRNA, in a manner mechanistically similar to pre-tRNA splicing.