PROJECT SUMMARY The marmoset monkey has emerged recently as a highly tractable nonhuman primate model of aging. Marmosets have relatively short lifespans, and exhibit similar aging-related declines to humans, including changes in blood pressure, immune function, and cognition. However, there is a biological “black box” in marmoset biology, the implications of which few researchers mention. Marmosets are natural chimeras, with siblings exchanging genetic material in utero. Some reports limit chimerism to only hematopoietic tissues (e.g., blood, spleen, bone marrow) whereas others suggest that other types of tissue can be chimeric, including germline tissues, with implications for heritability. It remains unclear how the presence of multiple genomes in an individual impacts marmoset physiology and the risk of age-related morbidities. The rationale for this study is that to understand the benefits and limitations of the marmoset model we must characterize the emergence and impacts of chimerism on an individual. These efforts are imperative to the overall objective of being able to predict and determine outcomes of chimerism. With the increasing appreciation that chimerism in humans is both widespread and a major driver of human health, marmoset models of chimerism may be a pivotal tool in aging research. Therefore, the specific aims for this proposal to 1) Define and map chimerism in marmosets, 2) Characterize the developmental dynamics of marmoset chimerism, and 3) Evaluate the costs of chimerism for aging marmosets.