GATA factors in hematopoiesis. Schematic representation of the requirements for GATA-1 and its cofactor FOG-1 during lineage specification and maturation of erythroid cells and megakaryocytes. Vertical bars represent maturation arrest observed in the absence of GATA-1 or FOG-1, as determined by murine gene targeting experiments. GATA-1 is required for differentiation of megakaryocytes and erythrocytes, as well as for mast cells and eosinophils.