The zinc finger transcription factor Ikaros is recognized as a key regulator of lymphocyte differentiation. Recently generated dominant negative mutants have hinted at a broader role in haematopoietic stem cell generation. Most recently, a mouse strain, IkarosPlastic, with a point mutation in Ikaros that disrupts DNA binding but preserves efficient assembly of Ikaros protein complexes, is embryonically lethal due to severe defects in erythrocyte differentiation (Papathanasiou P, et al,. Immunity, 2003). (1). These mice display normal murine globin gene expression in the fetal liver. However in humans the globin locus is under alternative regulatory control, particularly with respect to the fetal-to-adult globin switch. Thus, to determine if Ikaros plays a role in human globin switching we crossed the IkarosPlastic mice with mice transgenic for a YAC containing the entire human b-globin locus, which show human fetal to adult globin gene switching from E12 to E17. Embryos were harvested from E12.5 to E15.5 and globin expression was determined in the fetal liver by real-time PCR (relative to actin). At all time points human gamma-globin gene expression was not significantly altered by the presence of the IkarosPlastic mutatation (relative expression Ikaroswt/wt 1±0.11, IkarosPlastic/Plastic 0.82±0.12). In contrast, human beta-globin gene expression was significantly down-regulated in IkarosPlastic fetal livers (relative expression Ikaroswt/wt 1±0.14, IkarosPlastic/Plastic 0.18±0.07). Interestingly, neither murine a- or b-globin gene expression was significantly different to wild type mice, which suggests that the transcription factor Ikaros plays a specific role in the transcriptional activation of the human b-globin gene during development. The mechanism by which this occurs remains to be elucidated, however it is intriguing to consider that Ikaros may act as a potentiator of transcription for erythroid specific transcription factors such as EKLF. Experiments to address this will be presented.