hESCs (H1, XY, WiCell, and National Institutes of Healthapproved WA01) were maintained on mitomycin-blocked mouse embryonic fibroblast feeders in hESC growth medium (Dulbecco's modified Eagle's medium and Ham nutrient mixture F-12) supplemented with 15% knockout serum replacement enriched with 4 ng/ml of human basic fibroblast growth factor-2, 1ϫ nonessential amino acid, 1ϫ glutamax-I, and 1ϫ ␤-mercaptoethanol (all from Invitrogen Corp., Carlsbad, CA). Half of the medium was changed every 48 hours until the colonies were close to confluence. For differentiation induction, 2 to 2.5 ϫ 10 6 hESCs were resuspended in 3 ml of stem cell medium (HEScGro; Millipore Corp., Billerica, MA) supplemented with 50 ng/ml of vascular endothelial growth factor and 50 ng/ml of bone morphogenetic protein-4, plated in one well of a six-well plate (Ultra-Low; Corning Inc., Corning, NY), and incubated at 37°C with 5% CO 2 . After 24 hours, 40 ng/ml of stem cell factor, 40 ng/ml of thrombopoietin, and 40 ng/ml of Fms-related tyrosine kinase-3 (Flt3) ligand (R&D Systems, Inc., Minneapolis, MN) were added to the cultures, followed by 25 ng/ml each of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, IL-6, and IL-3, and 3 U/ml of human erythropoietin at day 3½ of differentiation culture.