Cell surface phenotype of embryonic and ESC-derived HSCs. Our data further defines the cell-surface phenotype of both murine ESC-HSCs and in vivo HSCs during ontogeny, especially with regard to the expression of the SLAM family of cell surface molecules. Similarly to early developmental HSC populations in the YS and AGM, ESC-HSCs express high levels of CD41. ESC-HSCs lack CD34 expression and are heterogeneous for both CD48 and CD45 cell-surface expression, suggesting that they represent a pool of phenotypically distinct repopulating cells. Surprisingly, we found that CD150 expression is developmentally regulated during HSC ontogeny; it is absent from E11.5 AGM and E12.5 placental HSCs but present on E14.5 FL and WBM HSC. CD48 was absent from HSCs throughout ontogeny.