['Reactive oxygen species (ROS) are involved in a wide range of cellular processes. However, few studies have examined the generation and function of ROS in human embryonic vascular development. In this study, the sources of ROS and their roles in the vascular differentiation of human embryonic stem cells (hESCs) were investigated.', 'During vascular differentiation of hESCs, CD34(+) cells had quiescence-related gene expression profiles and a large fraction of these cells were in G0 phase. In addition, levels of ROS, which were primarily generated through NOX4, were substantially higher in hESC-derived CD34(+) cells than in hESC-derived CD34(-) cells. To determine whether excess levels of ROS induce quiescence of hESC-derived CD34(+) cells, ROS levels were moderately reduced using selenium to enhance antioxidant activities of thioredoxin reductase and glutathione peroxidase. In comparison to untreated CD34(+) cells, selenium-treated CD34(+) cells exhibited changes in gene expression that favoured cell cycle progression, and had a greater proliferation and a smaller fraction of cells in G0 phase. Thus, selenium treatment increased the number of hESC-derived CD34(+) cells, thereby enhancing the efficiency with which hESCs differentiated into vascular endothelial and smooth muscle cells.'] Do reactive oxygen species regulate the quiescence of CD34-positive cells derived from human embryonic stem cells?