Abstract The goal of this project is to develop a mouse model that acquires human-like telomere homeostasis for the study of human aging, cancer, and other age-related diseases. In humans, most somatic cells lack telomerase expression and cannot replenish their telomeres. Telomeres progressively shorten upon successive cell divisions, functioning as an aging clock. Consequently, telomere attrition is a critical factor of human aging and telomere stabilization, predominantly via telomerase activation, is an essential event in the development of most human cancers. On the other hand, many other organisms, including laboratory mice, do not exhibit telomere-mediated replicative aging. Mice possess long telomeres and ubiquitous telomerase activity in adult tissues. This interspecies difference has become a bottleneck for addressing many fundamental questions in human aging and cancer biology using mouse models. To tackle this challenge, we have started to create a mouse strain with humanized telomere homeostasis. We have engineered a humanized mouse Tert allele (hmTert) by using regulatory sequences from the human TERT gene (hTERT) to replace their mouse counterparts. We found that the hmTert gene regulation recapitulated that of the hTERT gene during mouse development and in mouse adult tissues. Remarkably, the hmTert allele imposed a much shorter telomere length setpoint than the wildtype mTert gene in mice. In this application, we propose three specific aims: 1) Create a mouse strain with hmTert alleles and human-like short telomeres; 2) Study replicative aging in mice with humanized telomere homeostasis; and 3) Determine lifespan and health-span of mice with humanized telomeres.