We recently reported the importance of Synoviolin in quality control of proteins through the endoplasmic reticulum (ER)-associated degradation (ERAD) system and its involvement in the pathogenesis of arthropathy through its anti-apoptotic effect. For further understanding of the role of Synoviolin in vivo, we generated in this study synoviolin-deficient (syno(-/-)) mice by genetargeted disruption. Strikingly, all fetuses lacking syno died in utero around embryonic day 13.5, although Hrd1p, a yeast orthologue of Synoviolin, is non-essential for survival. Histologically, hypocellularity and aberrant apoptosis were noted in the syno(-/-) fetal liver. Moreover, definitive erythropoiesis was affected in non-cell autonomous manner in syno(-/-) embryos, causing death in utero. Cultured embryonic fibroblasts derived from syno(-/-) mice were more susceptible to endoplasmic reticulum stress-induced apoptosis than those from syno(+/+) mice, but the susceptibility was rescued by overexpression of synoviolin. Our findings emphasized the indispensable role of the Synoviolin in embryogenesis.