Bap31 and the related protein Bap29 are endoplasmic reticulum (ER) and ER-vesicle membrane proteins and members of the B-cell receptor-associated protein family. These two proteins are highly homologous and can form homo- and heterodimers. Bap31 is thought to be involved in the intracellular trafficking of several molecules such as MHC Class I molecules and CD11b/CD18. Bap31 may also play a role in the initiation of ER stress-induced apoptosis through its association with caspase-8 via a death effector domain in its cytoplasmic tail, possibly through the promotion of membrane fragmentation and the release of cytochrome c from mitochondria. Bap31 itself contains two caspase cleavage sites and is cleaved during apoptosis. The p20 fragment of Bap31, when expressed ectopically, is also a potent inducer cell death.