We have recently shown that Ire1 in Drosophila melanogaster cells independently mediates the cleavage and degradation of mRNAs encoding proteins that traverse the secretory pathway (Hollien and Weissman, 2006) . This new branch of the UPR, which we call regulated Ire1-dependent decay (RIDD), has the potential to selectively relieve the burden on the ER while clearing the translation and translocation machinery for the subsequent influx of new proteins induced by the UPR. Studies on the regulation of specific messages suggest that the RIDD pathway also operates in mammalian cells (Tirasophon et al., 2000; Iwawaki et al., 2001; Oikawa et al., 2007; Iqbal M aintenance of endoplasmic reticulum (ER) function is achieved in part through Ire1 (inositolrequiring enzyme 1), a transmembrane protein activated by protein misfolding in the ER. The cytoplasmic nuclease domain of Ire1 cleaves the messenger RNA (mRNA) encoding XBP-1 (X-box-binding protein 1), enabling splicing and production of this active transcription factor. We recently showed that Ire1 activation independently induces the rapid turnover of mRNAs encoding membrane and secreted proteins in Drosophila melanogaster cells through a pathway we call regulated Ire1-dependent decay (RIDD). In this study, we show that mouse fibroblasts expressing wild-type Ire1 but not an Ire1 variant lacking nuclease activity also degrade mRNAs in response to ER stress. Using a second variant of Ire1 that is activated by a small adenosine triphosphate analogue, we show that although XBP-1 splicing can be artificially induced in the absence of ER stress, RIDD appears to require both Ire1 activity and ER stress. Our data suggest that cells use a multitiered mechanism by which different conditions in the ER lead to distinct outputs from Ire1. ments of RIDD. By expressing wild-type and mutant variants of Ire1-, we find that the nuclease activity of Ire1 is required for both splicing and RIDD. However, these two outputs can be differentially triggered, revealing an unexpected complexity in Ire1 activation.