A model of the regulation of scl expression during primitive and definitive hematopoiesis. By the time scl starts to express, the PLPM has already bifurcated into angioblasts and erythroblasts. In erythroblasts, tbx16 is required for the expression of both scl-α and scl-β, which function redundantly during erythropoiesis. While in angioblasts, both scl isoforms are turned on separately by etsrp. Scl-α and fli1a act downstream of etsrp to specify angioblasts. At least some of these angioblasts have the potential to become hemogenic and acquire the competency for definitive hematopoiesis through an scl-β–dependent process. Vegf signaling, which is required for arterial induction, acts upstream of scl-β during definitive hematopoiesis.