Enables transcription cis-regulatory region binding activity. Acts upstream of or within several processes, including digestive system development; embryonic liver development; and hematopoietic stem cell differentiation. Predicted to be located in nucleus. Is expressed in several structures, including cardiovascular system; digestive system; musculature system; nervous system; and pleuroperitoneal region. Human ortholog(s) of this gene implicated in familial erythrocytosis 4; lung non-small cell carcinoma; polycythemia; and pulmonary hypertension. Orthologous to human EPAS1 (endothelial PAS domain protein 1).