Cotransfection of Id1 reduces CIITA-PIII activity in Raji B cells. Transient cotransfections of CIITA reporter plasmids PIIIfl and PIIIs-DelE with either 4 μg empty expression plasmid pRC/RSV (lanes 1 and 4) or 2 μg (lanes 2 and 5) and 4 μg (lanes 3 and 6) of Id1 expression plasmid were performed with Raji B cells. B cells were harvested after 48 hours and analyzed for luciferase activity. Luciferase activity values were normalized with Renilla luciferase activity values and represent the means of 3 experiments.