The placenta is a critical organ of mammalian embryogenesis, providing efficient nutrient, gas and waste exchange between the mother and fetus, as well as creating an immunologically privileged environment for the fetus and secreting hormones that maintain the pregnant state of the mother. The structure of the placenta is complex, consisting of regions of invasion and anchorage within the uterus as well as a region of vascular exchange, all of which have trophoblast cells as their primary structural and functional elements. The ability of the trophoblast cells to invade and form the placenta properly is crucial for successful development; placental dysfunction is associated with many disorders of pregnancy, including spontaneous abortions, intrauterine growth restriction, and preeclampsia, all of which are commonly associated with compromised placental vasculature. Trophoblast cells arise from the epithelial trophectoderm cells of the blastocyst and progressively differentiate into a variety of subtypes with specialized functions. Some of the factors regulating trophoblast specialization have begun to be identified, but the cellular and molecular mechanisms underlying differentiation and morphogenesis of the placenta are largely unknown. We have identified a novel gene involved in placental morphogenesis in the mouse, melanoma inhibitory activity 3 (Mia3), which encodes a protein involved in membrane transport and Golgi organization (TANGO1). A gene-trap mutation of this gene generated in ES cells, designated Xst199, leads to death of homozygous mutant embryos at day 10.5 of gestation, with severe defects in placental morphogenesis. The defect appears to originate at least as early as E8.5, as the chorion and ectoplacental cone are less well developed in the homozygous mutants than in heterozygous and wild type litter mates. TANGO1 in cultured human cells functions to facilitate loading of COPII coated secretory vesicles with specific cargo proteins, thus I hypothesize that the Xst199 mutation of Tango1 in mouse embryos interferes with secretion of proteins specifically required for trophoblast differentiation, leading to placental dysfunction. In this pilot study I propose to further characterize the phenotype of this mutation and the normal function of Tango1 in trophoblast cells, and determine the importance of this novel pathway to the regulation of trophoblast cell differentiation.