Correlation of major events in hematopoietic development with expression of flk-1, scl, and LMO2. Gastrulating epiblast cells give rise to ventral mesoderm under the influence of BMP-4. Putative hemangioblasts expressing flk-1 and scl differentiate down endothelial and hematopoietic pathways. The molecular event(s) (gene x), which specifies a hematopoietic progenitor and downregulates flk-1, is not known, although LMO2 is a possible candidate. Differentiating erythroid lineage cells continue to express scl and LMO2 but their expression is extinguished in myeloid cells.