Growth factor-independence 1b (Gfi1b) is a zinc finger transcription factor 
essential for erythroid and megakaryocytic development. To better understand 
Gfi1b regulation and to know the implication of the level of expression of this 
gene in human pathology, we have searched for promoter punctual sequence 
variations in 214 patients with different hematological diseases. We found two 
previously unknown congenital mutations at evolutionary conserved GATA and 
octamer-binding (Oct) transcription factor sites. The Oct site mutation was also 
found in five relatives of the patient. The GATA motif mutation reduced promoter 
activity by 50% in vitro, while homozygous patients with the octamer site 
mutation showed a four-to-five times increase of Gfi1b RNA in platelets. 
Electrophoretic mobility shift analyses demonstrated that different protein 
complexes bind to both sites and that binding is reduced by the mutations. 
Finally, we found that GATA-1 and Oct-1 are the main components of each complex. 
This study provides evidences of a new mechanism for Gfi1b repression. This is 
also the first report of Gfi1b mutations with a functional implication; further 
investigation and follow-up will clarify the involvement of these mutations in 
hematological disease.