ICSBP and IRF-4 synergistically downregulate the expression of the ISG15 promoter. K562 cells were transfected with the luciferase expression vector pGL-ISG15p and pcDNA-IRF-4 and/or pcDNA-ICSBP. The graph shows the results of 3 independent experiments. The values were standardized relative to the internal control pRL-TK (see Materials and Methods).