Transcription factor that acts synergistically with lmo2 and gata1 to specify embryonic dorsal mesoderm to a hematopoietic fate. First expressed in patches on the ventral side of the embryo in a region that will give rise to hematopoietic tissue. By late neurula stages, expressed throughout the ventral blood island region. By tailbud stages, expression extends to probable vascular progenitor cells, but is excluded from the presumptive liver anlage. Also expressed in the central nervous system at the tailbud stage. First expressed at stage 12/13 (gastrula) of embryogenesis, with expression increasing through the tadpole stages. By bmp-signaling in the ventral mesoderm. Inhibition by fgf regulates the timing of hematopoiesis.