THE zinc-finger transcription factor GATA-1 (previously known as GF-1, NF-E1 or Eryf 1 (refs 1-5)) binds to GATA consensus elements in regulatory regions of theα- and β-globin gene clusters^2–6 and other erythroid cell-specific genes^7–9. Analysis of the effects of mutations in GATA-binding sites in cell culture and in binding assays in vitro^2,5,10,11, as well as transactivation studies with GATA-1 expression vectors in heterologous cells¹², have provided indirect evidence that this factor is involved in the activation of globin and other genes during erythroid cell maturation. GATA-1 is also expressed in megakaryocytes^13,14 and mast cells¹³, but not in other blood cell lineages or in non-haemopoietic cells. To investigate the role of this factor in haematopoiesis in vivo. we disrupted the X-linked GATA-1 gene by homologous recombination in a male (XY) murine embryonic stem cell line and tested the GATA-1-deficient cells for their ability to contribute to different tissues in chimaeric mice. The mutant embryonic stem cells contributed to all non-haemopoietic tissues tested and to a white blood cell fraction, but failed to give rise to mature red blood cells. This demonstrates that GATA-1 is required for the normal differentiation of erythroid cells, and that other GATA-binding proteins^15,16 cannot compensate for its absence.