Holoprosencephaly (HPE), a common malformation of the forebrain and midface, is caused by failure to define the early rostroventral midline. HPE has a complex etiology, and is associated with both genetic and environmental risk factors. Among the environmental factors implicated in elevated risk of HPE is prenatal alcohol exposure. Heterozygous mutations in the Nodal and Hedgehog (HH) signaling pathways are associated with HPE. However, clinical presentation of HPE is highly variable, and many mutation carriers are largely unaffected. These and additional observations have led to a multifactorial model, in which the outcome associated with a mutation is influenced by more common genetic variants and/or environmental exposures. HPE is therefore an excellent model system for understanding gene-environment interactions and the multifactorial etiology of many common birth defects. We have modeled this scenario in mice. CDON encodes a multifunctional, cell surface coreceptor that promotes signaling by several pathways, including the HH pathway. Mice with a mutation in Cdon have only a minor deficit in HH signaling but, unlike wild type mice, are sensitive to induction of a full range of HPE defects by transient exposure to ethanol (EtOH) during early embryogenesis. Despite many studies employing this and related models, the embryonic cell types and developmental events that are direct targets of EtOH’s teratogenicity in HPE remain unidentified. The Nodal pathway lies developmentally upstream from HH in rostroventral midline patterning. Our recent findings are consistent with the hypotheses that: 1) Cdon functions cell autonomously to regulate Nodal pathway signaling in cells of the anterior primitive streak (APS); and 2) inhibition of Nodal signaling in APS cells is the target of synergy between fetal EtOH and Cdon mutation. To test these hypotheses, the following aims are proposed: 1) to determine if CDON functions cell-autonomously in APS cells in EtOH-induced HPE, we will use conditional mutagenesis to remove Cdon in the mouse APS or alternative structures and assess the sensitivity of the mice to EtOH-induced HPE. Embryos will be analyzed at multiple stages for HPE phenotypes by morphological analyses, in situ hybridization and qRT-PCR for alterations in gene expression, and assays for cell proliferation and apoptosis; and 2) to determine mechanisms of EtOH action in HPE, we will study mouse epiblast stem cells as an in vitro model for APS cells. We will study responses of these cells to EtOH by transcriptomic and signal transduction analyses. Findings from these experiments will then be applied to studies of EtOH-treated embryos. The two aims are synergistic in that they merge developmental genetics and mechanistic molecular analyses in approaching the problem of how fetal alcohol exposure contributes to common features of HPE and related brain and craniofacial defects.