Nucleotides 410-633 of a human XBP1 cDNA including a Kozak sequence and FLAG-tag coding sequence was fused to the venus fluorescent reporter and then placed under the control of the CAG promoter construct in the pCAX expression vector (pCAX is a variant of pCAGG). The XBP1 fragment lacks the DNA binding domain but includes a 26-nt endoplasmic reticulum stress specific intron. Under normal conditions, the mRNA of the fusion gene is not spliced and translation terminates at a stop codon near the junction of XBP1 and venus. During ER stress, the 26 nt intron is spliced out and translation terminates at the stop codon of venus.