The differentiation of dopaminergic (DA) neurons from mouse embryonic stem cells (ESCs) can be efficiently induced, making these neurons a potential source for transplantation as a treatment for Parkinson's disease, a condition characterized by the gradual loss of midbrain DA neurons. One of the major persistent obstacles to the successful implementation of therapeutic ESC transplantation is the propensity of ESC-derived grafts to form tumors in vivo. To address this problem, we used fluorescence-activated cell sorting to purify mouse ESC-derived neural precursors expressing the neural precursor marker Sox1. ESC-derived, Sox1+ cells began to express neuronal cell markers and differentiated into DA neurons upon transplantation into mouse brains but did not generate tumors in this site. In contrast, Sox1− cells that expressed ESC markers frequently formed tumors in vivo. These results indicate that Sox1-based cell sorting of neural precursors prevents graft-derived tumor formation after transplantation, providing a promising strategy for cell transplantation therapy of neurodegenerative disorders.