Using comprehensive genetic studies on neuronal stem/progenitors cells through genome-wide screening with oligonucleotide arrays, we identified an endoplasmic reticulum (ER) -resident protein, Tweety homologue 1 (ttyh1). Ttyh1 encodes a glycosylated protein composed of five predicted transmembrane segments and a C-terminus that is enriched in negatively charged residues capable of Ca(2+) binding. Ttyh1-containing membranes changed to segmented tubuloreticular structures during mitosis, suggesting that the ER-containing Ttyh1 could be responsible for Ca(2+) sequestration and Ca(2+) concentration regulation during mitosis. Ttyh1 inactivation in mice resulted in early embryonic lethality before organization of the nervous system, revealing that ttyh1 is essential in murine embryonic development. Our findings indicate that Ttyh1 plays an indispensable role during mitosis in early embryogenesis, possibly by maintaining Ca(2+) homeostasis in the ER.