SUMMARY Despite their emergence from distal esophagus and distal stomach, respectively, esophageal adenocarcinoma (EAC) and intestinal gastric cancer (iGC) share the natural histories and molecule genetics of a single disease. Historically, EAC and iGC were among the first cancers to be linked to the prior presence of discrete, pre- cancerous lesions that can progress to dysplasia and then invasive disease over a two-decade interval. In both cases the earliest precancerous lesion was an odd "intestinal metaplasia; IM" known as "Barrett's esophagus (BE)" for EAC and "gastric intestinal metaplasia (GIM)" for iGC. These common evolutionary features have been extended by cancer genetics breakthroughs that place EAC and iGC into single cluster distinct from other gastric and esophageal cancers. While it was widely anticipated that advances in endoscopic and ablative technologies applied to precursor lesions would spell the end of EAC and iGC, rates of EAC and iGC have not appreciably decreased and most patients still present with advanced disease and poor five-year survival. This dire clinical reality has predicated a broad effort to understand the cell-of-origin of these diseases, their earliest emergence towards pathology, as well as their detection and pharmaceutical elimination. A highly collaborative team consisting of upper gastrointestinal oncologists, stem cell and molecular biologists, experts in murine cancer modeling, and proteomics specialists has employed advanced stem cell cloning technologies to capture patient-matched stem cells in each of the successive lesions in patients with EAC and iGC. In addition to the high-resolution phylogenetics afforded by these stem cells, this analysis has revealed the BE and GIM stem cells are indistinguishable at the level of whole genome expression profiling down to the level of homeotic transcription factors that define cellular identity. Common cell surface markers of BE and GIM stem cells identify a discrete population of cells at both the gastroesophageal (GE) junction and in the distal stomach of normal mice which we hypothesize are the intrinsic source of the IM for EAC and iGC respectively. These markers have also enabled the cloning of the corresponding site-specific stem cells, which we find to be indistinguishable gene expression profiles and to be committed to IM upon in vitro differentiation. In three aims, we will 1) use similar methods to clone the intrinsic IM stem cells from human fetal and adult GE junctions and gastric mucosa; 2) engineer mouse models for conditional expression of oncogenic factors in intrinsic IM cells; and 3) identify small molecules that selectively target intrinsic IM stem cells as leads for therapeutics to prevent EAC and iGC. We anticipate that the studies proposed herein will provide new insights into the biology and origin of these remarkably similar and widespread cancers, provide datasets essential for prospective early detection screens, and yield highly se...