Stem cells appear to possess great plasticity, but the cellular mechanisms regulating their behavior and fate are not understood. If these mechanisms can be harnessed to obtain cells specifically required for therapy, diagnosis or drug discovery, it may be possible to restore function to tissues and organ systems that have been compromised by congenital disorders, developmental malfunction, age, injury, disease or drug exposure. The National Institute of Neurological Disorders and Stroke (NINDS), the National Institute on Deafness and Other Communication Disorders (NIDCD), the National Institute on Aging (NIA), the National Institute of Child Health and Human Development (NICHD), the National Cancer Institute (NCI), the National Institute of Drug Abuse (NIDA), and the National Institute of Mental Health (NIMH) invite applications for studies on the characterization, behavior and plasticity of human and non-human stem cells, regulation of their replication, differentiation, integration and function in the nervous system, and the identification and characterization of normal and tumor stem cells. An understanding of intrinsic and extrinsic signals, especially those involved in the stem cell niche, age-dependent processes and genetic factors that govern the activities of pluripotent cells is crucial in order to utilize them to develop safe and effective treatments for the restoration of function, or to prevent their transformation into tumor-generating cells. Although animal studies demonstrate that stem or progenitor cells can be derived from a variety of tissues and from hosts of different ages, the requirements and potential for differentiation of each type of pluripotent cell appear to be unique. We lack a clear understanding of the intrinsic properties that distinguish one population from another, and how these populations differ in their response to similar conditions in vitro and in vivo. This Program Announcement, which replaces PA-01-078 (Biology of Non-Human Stem Cells in the Environment of the Nervous System) and PA-02-025 (Plasticity of Human Stem Cells in the Nervous System), encourages applications to study the fundamental properties of all classes of human and non-human stem cells, and to confirm, extend, and compare the behavior of stem cells that are derived from different sources and ages or exposed to different regimes in vitro and in vivo or derived from tumors. Of high priority are studies to develop methods for identifying, isolating and characterizing specific precursor populations at intermediate stages of differentiation into neurons and glia, and their relationship to tumor-generating cells. Projects that address comparisons between different classes of human stem cells and between human and non-human stem cells would also be directly relevant to this PA.