Telomerase serves a crucial function in human cancer by elongating telomeres and supporting cancer cell immortalization. Telomerase is thought to be upregulated during human cancer progression, enabling aspiring cancer cells to overcome senescence barriers. Telomerase expression in normal tissues is highly enriched in stem cells and progenitor cells, and is efficiently silenced in more differentiated progeny. Despite clear evidence for restricted expression of telomerase, challenges in detecting telomerase at the single cell level have limited our ability to understand telomerase regulation intissues and in cancers in depth. In addition to its role in telomere elongation, we have shown that TERT, the telomerase protein subunit, exerts potent effects on tissue stem cells through an auxiliary pathway. TERT modulates the output of the Wnt signaling pathway through an interaction with the chromatin remodeling protein Brg1, and a direct association with promoter chromatin at Wnt-regulated genes. TERT overexpression activates quiescent stem cells in vivo and regulates a Wnt-related gene expression program. We hypothesize that telomerase is almost universally associated with progenitor cells and with human cancers, not only to overcome senescence barriers, but also to support Wnt signaling and associated programs of self-renewal, proliferation and survival critical for progenitor cells and for developing cancers. o address these hypotheses, we have generated novel mouse knockin and knockout reagents that facilitate the dissection of endogenous telomerase at the single cell level and at the proteinlevel, and that allow deletion of TERT in vivo in tissues and in cancers with spatiotemporal control. Remarkably, we find that acute deletion of TERT in vivo abrogates Wnt signaling and leads to immediate phenotypes not evident in germline telomerase knockout mice. We plan to pursue the following specific aims: (1) To define the identity and function of telomerase-positive cells in tissues (2) To understand the role of TERT in epithelial cancer through acute deletion and (3) To understand the function of TERT in controlling epigenetic tumor states.