To determine whether the RIDD pathway functions in mammals as well as in Drosophila, we used a mouse embryonic fibroblast (MEF) cell line established by Lee et al. (2002) in which the Ire1- gene has been disrupted. We stably introduced human Ire1- (hIre1) into these cells using flippasemediated, site-directed recombination , which allowed us to insert the wild-type and hIre1 variants discussed in the next sections into the same sites within the genome. In response to various forms of chemically induced ER stress, the reconstituted cells (referred to here as hIre1 R ) but not the Ire1 knockout (Ire1 / ) cells induced the splicing of XBP-1 et al., 2008; Lipson et al., 2008) . Mammals express two isoforms of Ire1: Ire1- is expressed ubiquitously, and Ire1- is expressed in intestinal epithelial cells. Overexpression of Ire1- leads to cleavage of its own message in COS-1 cells (Tirasophon et al., 2000) and reduced levels of the message encoding CD59 (complement defense 59) in HeLa cells (Oikawa et al., 2007) . Ire1- also appears to mediate the degradation of insulin transcripts in pancreatic  cells under chronic high glucose conditions, perhaps promoting cell survival during extreme chronic stress (Lipson et al., 2008) . Ire1- appears to have alternative targets as well, mediating cleavage of the 28-S ribosomal RNA (Iwawaki et al., 2001) and the mRNA encoding microsomal triglyceride transfer protein, an ER chaperone important for the assembly of lipid transport vesicles (Iqbal et al., 2008) . Together, these examples of Ire1 function in mRNA decay may explain observations that Ire1 in metazoans has a broader range of physiological outputs than XBP-1 splicing (Zhang et al., 2005) .