Trisomy 21 (Down syndrome, DS), is among the most complicated genetic situations compatible with substantial survival. The clinical presentation of DS represents the interaction of many triplicated genes throughout development. Understanding what individual genes do is a necessary component of approaches to therapy for features of Down syndrome, but it is not sufficient. The earlier periods of this award focused on creating and characterizing animal models in which to study DS, supporting assessment of all tissues at all stages of life. In the last award period we used these models and principles to make three significant advances. First, we determined that trisomic mice recapitulate (and predict) structural problems observed in the very small DS cerebellum, defined the timing, cell type, process and growth factor (SHH) responsible for this hypocellularity and then cured it in mice. Second, we provided the first experimental evidence that DS is a neurocristopathy by showing that the craniofacial hypoplasia in DS and in mouse models originates with problems in delamination, migration and proliferation of neural crest cells (NCC) in the first pharyngeal arch. Third, we provided biological evidence to answer a 50 year old statistical argument that people with DS get substantially (90%) less cancer than do euploid individuals and identified a single gene, Ets2, dosage for which is inversely correlated with intestinal tumor number in a model of colon cancer. Because NCC and SHH each affect many (overlapping) aspects of development, we will test the hypothesis that they represent common denominators of DS phenotypes. We will use a pharmacological approach to cure the NCC deficit leading to craniofacial hypoplasia. We have begun a survey of the entire Hsa21 gene set to determine gene dosage effects on early development in zebrafish. We will further characterize gene expression with regard to nuclear compartmentalization, a newly appreciated epigenetic regulatory mechanism. We will define more precisely the mechanism of Ets2 tumor repression, and screen for drugs that might act on this pathway as a prophylactic for cancer in everyone, regardless of ploidy.