Eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER). In this case, so-called unfolded protein response (UPR) genes are induced. We determined the transcriptional expression ofArabidopsis thaliana UPRgenes by fluid microarray analysis of tunicamycin-treated plantlets. Two hundred and fifteen up-regulated genes and 17 down-regulated ones were identified.