DDK syndrome is the polar-lethal embryonic death that occurs at the morula-blastocyst transition when female mice of the DDK strain are mated with males from many other inbred strains (so-called alien males). Embryonic death is caused by incompatibility between a DDK oocyte factor and an alien male gene, both of which map to the Om locus on mouse chromosome 11. We compared global transcription patterns of DDK x DDK embryos (high viability) and DDK x C57BL/6 embryos (low viability) at the morula stage, approximately 24 h before any morphological manifestations of DDK syndrome are observed. Of the transcripts that are differentially more abundant in the DDK x C57BL/6 embryos, many are the products of genes induced by the "unfolded protein response." We confirmed by quantitative RT-PCR that a number of genes in this pathway are upregulated in the DDK x C57BL/6 embryos. Immunostaining of the endoplasmic reticulum (ER) marker BIP/GRP78 (immunoglobin-binding protein/glucose-regulated protein of 78 kDa), official symbol HSPA5, heat shock protein 5 revealed an accompanying abnormal HSPA5 accumulation and ER structure in the DDK x C57BL/6 embryos. Immunostaining for HERPUD1 (homocysteine-inducible, ER stress-inducible, ubiquitin-like domain member 1) and ATF4 (activating transcription factor 4) also revealed accumulation of these stress-response products. Our results indicate that the unfolded protein response is induced in embryos destined to die of DDK syndrome and that the embryonic death observed is associated with inability to resolve the associated ER stress.