Final answer:
IRE1 splices out a 26 base intron from the mRNA encoding for XBP1, which enables the translation of a potent form of this transcription factor to help alleviate stress within the ER.
Step-by-step explanation:
The question is asking about a specific function of the IRE1 protein, which is involved in the cellular stress response pathway known as the unfolded protein response (UPR). In response to the accumulation of unfolded proteins, IRE1 excises an intron from the mRNA encoding for a transcription factor involved in alleviating stress within the endoplasmic reticulum (ER). The correct answer is B) 26; XBP1. IRE1 splices out a 26 base intron, and this splicing event shifts the reading frame to allow the production of a potent transcription factor, XBP1, which then goes on to upregulate the expression of genes that help to restore normal function to the ER.