Transcription factor that plays a pivotal role in hemopoietic and endothelial development, acting synergistically with lmo2 and downstream of clo. Specifies mesodermal precursors to a hemangioblast cell fate. Hemangioblasts are bipotential precursors of blood and endothelium, and in the absence of hemopoietic induction cues such as gata1, tal1/scl-lmo2-induced hemangioblasts differentiate into endothelial cells. Isoform alpha and isoform beta are redundant for the initiation of primitive hemopoiesis but have distinct roles in the regulation of primitive erythroid differentiation and definitive hemopoietic stem cell specification, most likely due to differences in expression levels. Specification of definitive hemopoietic stem cells requires isoform beta. DNA binding is required for erythroid maturation, but not for its other hemopoietic functions. Endothelial roles include development of the dorsal aorta, the site of definitive hemopoiesis in the embryo. Required for angiogenesis but not angioblast specification. Has an additional role in endocardium formation during heart development. May play a role in central nervous system development.