This proposal is to request support for a Keystone Symposia meeting entitled "Tumor Suppressors and Stem Cell Biology", organized by Sean J. Morrison and Alejandro S nchez Alvarado, which will be held in British Columbia, Canada from February 24 - 29, 2008. Stem cells play key roles in the development, maintenance, and reproduction of all known multicellular organisms. Stem cells operate in a wide variety of biological contexts from homeostasis to regeneration and must be subject to complex and robust regulation. Recent evidence suggests that tumor suppressor mechanisms, originally discovered for their ability to inhibit tumorigenesis, also play widespread roles in the physiological regulation of stem cells. Tumor suppressors play critical roles in the regulation of tissue homeostasis, stem cell self-renewal, regeneration, and aging. This reveals mechanistic links between the regulation of stem cells, and the regulation of cancer cells that could potentially be exploited to target cancer and to enhance regeneration. The dual roles of these mechanisms in stem cell and cancer biology illuminate the evolved balance between the promotion of regeneration and the suppression of cancer. [unreadable] [unreadable] [unreadable] [unreadable]