Multipotent neural stem cells are present throughout the development of the central nervous system (CNS), persist into adulthood in defined locations and can be derived from more primitive embryonic stem cells. We show that SOX2, an HMG box transcription factor, is expressed in multipotent neural stem cells at all stages of mouse ontogeny. We have generated transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous locus-regulatory regions of the Sox2 gene to prospectively identify neural stem/progenitor cells in vivo and in vitro. Fluorescent cells coexpress SOX2 protein, and EGFP fluorescence is detected in proliferating neural progenitor cells of the entire anterior-posterior axis of the CNS from neural plate stages to adulthood. SOX2-EGFP cells can form neurospheres that can be passaged repeatedly and can differentiate into neurons, astrocytes and oligodendrocytes. Moreover, prospective clonal analysis of SOX2- EGFP-positive cells shows that all neurospheres, whether isolated from the embryonic CNS or the adult CNS, express SOX2-EGFP. In contrast, the pattern of SOX2-EGFP expression using randomly integrated Sox2 promoter/reporter construct differs, and neurospheres are heterogeneous for EGFP expression. These studies demonstrate that SOX2 may meet the requirements of a universal neural stem cell marker and provides a means to identify cells which fulfill the basic criteria of a stem cell: self-renewal and multipotent differentiation.