A model of hematoendothelial differentiation in hESC/OP9 coculture. Hematopoietic and endothelial cells develop from early precursors identified by a CD34⁺KDR⁺(CD43^–) phenotype. These precursors appear at day 3 of differentiation and retain hematoendothelial potential up to day 5, but after 6 days, CD34⁺CD43^– KDR⁺ cells constitute a population of committed endothelial cells (ECs). CD43 is identified as a specific marker of early hematopoietic progenitors. Two types of CD43⁺ hematopoietic progenitors are identified in hESC/OP9 coculture: (1) CD43⁺CD41a/CD235a⁺ erythro-megakaryocytic progenitors (E/Mk-HP) first detectable on day 4 of differentiation, and (2) CD43⁺CD41a/CD235a^– multilineage (mLin) progenitors (HP1) appeared 2 days later. Emergence of E/Mk-HP before mLin HP1 and residual expression of VE-cadherin, FLT1, and CD105 endothelial markers by HP1 cells may reflect a step-wise endothelial commitment of CD34⁺KDR⁺ hematoendothelial precursors (block arrow): CD34⁺KDR⁺ precursors at initial pre-endothelial commitment stage are only competent to generate E/Mk-HP through CD235⁺ intermediates, whereas multipotent HP1 are derived from CD34⁺KDR⁺ precursors with an endothelial phenotype (VE-cadherin^highCD105^high). HP1 have lymphomyeloid potential and a gene expression profile found in the most immature hematopoietic progenitors. HP1 transition to CD45⁺ stage (HP2) is associated with progressive myeloid commitment and a decrease of lymphoid potential. CD34⁺CD43^–KDR^–CD26^high cells arise along with the first CD34⁺KDR⁺ cells and may comprise more than 20% of total CD34⁺ cells in hESC/OP9 cocultures. These cells are devoid of detectable hematoendothelial potential.