Transcription factor GATA-1 is essential for the development of erythroid cells and megakaryocytes. Each of its 2 zinc fingers is critical for normal function. The C-terminal finger is necessary for DNA binding. The N finger mediates interaction with FOG-1, a cofactor for GATA-1, and also modulates DNA-binding affinity, notably at complex or palindromic GATA sites. Residues of the N finger-mediating interaction with FOG-1 lie on the surface of the N finger facing away from DNA. Strong sequence conservation of residues facing DNA suggests that this other surface may also have an important role. We report here that a syndrome of X-linked thrombocytopenia with thalassemia in humans is caused by a missense mutation (Arg216Gln) in the GATA-1 N finger. To investigate the functional consequences of this substitution, we used site-directed mutagenesis to alter the corresponding residue in GATA-1. Compared with wild-type GATA-1, Arg216Gln GATA-1 shows comparable affinity to single GATA sites but decreased affinity to palindromic sites. Arg216Gln GATA-1 interacts with FOG-1 similarly with wild-type GATA-1. Arg216Gln GATA-1 supports erythroid maturation of GATA-1 erythroid cells, albeit at reduced efficiency compared with wild-type GATA-1. Together, these findings suggest that residues of the N finger of GATA-1-facing DNA contribute to GATA-1 function apart from interaction with the cofactor FOG-1. This is also the first example of beta-thalassemia in humans caused by a mutation in an erythroid transcription factor.