Human embryonic stem cells (hESCs) are derived from the inner cell mass of preimplantation embryos; they can be cultured indefinitely and differentiated into many cell types in vitro. These cells therefore have the ability to provide insights into human disease and provide a potential unlimited supply of cells for cell-based therapy. Little is known about the factors that are important for maintaining undifferentiated hESCs in vitro, however. As a tool to investigate these factors, transfected hES clonal cell lines were generated; these lines are able to express the enhanced green fluorescent protein (EGFP) reporter gene under control of the OCT4 promoter. OCT4 is an important marker of the undifferentiated state and a central regulator of pluripotency in ES cells. These OCT4-EGFP clonal cell lines exhibit features similar to parental hESCs, are pluripotent, and are able to produce all three embryonic germ layer cells. Expression of OCT4-EGFP is colocalized with endogenous OCT4, as well as the hESC surface antigens SSEA4 and Tra-1-60. In addition, the expression is retained in culture for an extensive period of time. Differentiation of these cells toward the neural lineage and targeted knockdown of endogenous OCT4 expression by RNA interference downregulated the EGFP expression in these cell lines, and this correlates closely with the reduction of endogenous OCT4 expression. Therefore, these cell lines provide an easy and noninvasive method to monitor expression of OCT4 in hESCs, and they will be invaluable for studying not only OCT4 function in hESC self-renewal and differentiation but also the factors required for maintenance of undifferentiated hESCs in culture.