Functions as part of the SSP (stage selector protein) complex, a complex that contributes to the preferential expression of the gamma-gene in fetal erythroid cells by facilitating the interaction of the gamma-globin genes with enhancer elements contained in the locus control region (LCR). The complex binds to the stage selector element (SSE) in the proximal gamma-globin promoter. In contrast, isoform 2 acts as a repressor of gamma- globin gene expression by preventing NFE2 and RNA polymerase II recruitment to the promoter.