To the Editor: The molecular mechanisms and the control of smooth muscle cell (SMC) differentiation have been extensively investigated because of its therapeutic potential.1 To date, different cell types have been used to study SMC differentiation, including a variety of mouse embryonic stem cells,2 adult stem cells,3,4 and others.5 Because several fundamental differences exist between mouse and human embryonic development,6 lack of a good model system to study human SMC differentiation has hampered the progress of translating SMC knowledge to novel clinical therapies. Human embryonic stem (hES) cells provide a valuable source of cells for studying human cell differentiation and developing therapeutic potentials in regenerative medicine. Since the initial report describing the derivation of hES cells,7 a variety of studies have established in vitro differentiation strategies to several lineages. Recently, it has been demonstrated that vascular progenitors derived from hES cells could be differentiated into endothelial cells and SMCs by endothelial …