The endothelial-specific receptor tyrosine kinase flt-1 (VEGFR-1) is expressed early on during endothelial lineage commitment both in vivo and in vitro. However, the exact function of flt-1 in vascular development still remains unclear. Here we report that a 2.2-kb fragment 5' of the mouse flt-1 gene becomes transcriptionally active during endothelial cell differentiation in developing embryoid bodies derived from mouse ES cells. Reporter gene expression correlated well with PECAM-1 expression and mirrored the expression pattern of the endogenous flt-1 gene. The temporal and spatial activity of the 2.2-kb flt-1 promoter provides a means to (1) identify a living population of early committed endothelial/bipotential progenitors and (2) ectopically express biologically active genes during lineage commitment,