Ink4b promoter activity is positively regulated by PU.1 and ICSBP and can be further increased by addition of IFNβ. M1 cells, cultivated in the presence or absence of IFNβ, were transiently cotransfected with the -444/-117 bp promoter construct as well as expression plasmids for PU.1 and/or ICSBP or the corresponding empty vector and analyzed for promoter activity. Shown are relative values normalized for Renilla luciferase activity. Error bars depict standard error.