Self-renewal and repair of tissues in adult mammals is fueled by rare tissue stem cells. Identification and subsequent study of such adult stem cells depends strictly on the availability of molecular markers. To date, a paucity of single definitive markers has complicated the study of the biology of these rare cells. Our interest in the role of the Wnt pathway in intestinal homeostasis and cancer has led us to exhaustively define the Wnt target gene program shared between these two processes. Within this program, the cell surface receptor-encoding Lgr5 represented a candidate stem cell marker for the intestine. The creation of Lgr5-genetic mouse models subsequently allowed us to definitively identify the Lgr5+ cells as the intestinal stem cells. These stem cells divide every day, yet persist throughout life. Using the same mouse models, we have found that Lgr5 marks stem cells in a variety of other tissues. For two family members with similarly restricted expression domains, Lgr4 and -6, comparable genetic tools have been generated. In this proposal, I aim to exploit these unique genetic tools to study primary adult stem cells in health and disease. This should allow us to outline unique and common characteristics of individual stem cell types, and possibly- define the minimum molecular determinants of stemness. Specifically, I aim: 1. To make a complete inventory of adult stem cells expressing Lgr4, Lgr5 and/or Lgr6 using the transgenic mouse models. 2. To FACS-purify stem cells from selected tissues, and detail their molecular and metabolic characteristics. 3. To genetically define the biological role of the Lgr-family receptors in adult stem cells by gene knockout. 4. To translate these findings to the equivalent human tissues using antibody-based histology and FACS technology. 5. To explore the usefulness of the Lgr proteins as markers of cancer stem cells in human solid tumors.