Mesenchymal stem cells (MSCs) have been presumed to include a subpopulation of pluripotent-like cells as they differentiate not only into the same mesodermal-lineage cells but also into ectodermal- and endodermal-lineage cells and exert tissue regenerative effects in a wide variety of tissues. A novel type of pluripotent stem cell, Multilineage-differentiating stress enduring (Muse) cells, was recently discovered in mesenchymal tissues such as the bone marrow, adipose tissue, dermis and connective tissue of organs, as well as in cultured fibroblasts and bone marrow-MSCs. Muse cells are able to differentiate into all three germ layers from a single cell and to self-renew, and yet exhibit non-tumorigenic and low telomerase activities. They can migrate to and target damaged sites in vivo, spontaneously differentiate into cells compatible with the targeted tissue, and contribute to tissue repair. Thus, Muse cells may account for the wide variety of differentiation abilities and tissue repair effects that have been observed in MSCs. Muse cells are unique in that they are pluripotent stem cells that belong in the living body, and are thus assumed to play an important role in ‘regenerative homeostasis’ in vivo.