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. Expressed in hemopoietic and endothelial lineages. Isoform beta emerges first, expressing in the entire anterior and posterior lateral mesoderm (ALM and PLM respectively), and in the ventral wall of the dorsal aorta, where definitive hemopoiesis begins. Isoform alpha expresses later as two pairs of stripes in the PLM and ALM, and becomes restricted to the intermediate cell mass (ICM) by the 18-somite stage. The ICM is the key site of primitive hemopoiesis, giving rise to the erythroid lineage. Also expressed in all stages of endocardial cell migration and in the developing midbrain, hindbrain and spinal cord. In adults, expressed in the main hemopoietic organs, namely the kidney (where isoform alpha is the predominant isoform) and the spleen. Also expressed in the liver, gill and gonads. Expression of isoform beta begins at the 1-2 somite stage, peaks at the 11-18 somite stage, and is maintained at a lower level in the adult kidney. Isoform alpha isn't expressed until the 4-somite stage, after which expression levels rapidly increase. Truncated N-terminus.