Functional assays indicate that the XG upstream GATA motif binds GATA1 and can enhance transcription. (A) Electrophoretic mobility shift assays were performed with 35-bp biotinylated probes spanning the rs311103 SNP. Only the wild-type probe exhibited a shift (black arrowhead) upon incubation with nuclear extract from K562 cells and a further supershift (white arrowhead) with addition of anti-GATA1, highlighting its affinity for GATA1. Preincubation with 200-fold unlabeled probes abolished the mobility shifts, indicating specificity. The figure is representative of 3 independent experiments. (B) HEL cells were transfected with plasmids carrying the wild-type or mutant XG GATA sequence and a luciferase gene driven by the ABO promoter (n = 9). The relative luciferase activity obtained with the pGL3-SN vector was used as reference and normalized to 1. Data represent mean values; error bars indicate standard errors of the mean. ***P < .001.