Blood group Duffy gene (FY) promoter in Duffy-negative individuals contains a point mutation in the GATA1 protein-binding motif, which was suggested to be responsible for erythroid suppression of FY. We developed two transgenic mouse lines with FY from both Duffy phenotypes. Transgenic mice with FY from Duffy-positive phenotype expressed Duffy protein both in red blood cells (RBCs) and non-erythroid tissues. Transgenic mice with FY from Duffy-negative phenotype did not express Duffy protein in RBCs, but it was expressed in non-erythroid tissues. This is the first in vivo experimental evidence showing the effect of -33T-->C promoter mutation on FY expression.