PU.1 and ICSBP are both present at the Ink4b promoter region. Chromatin immunoprecipitation (ChIP) analysis was performed with M1 cells cultivated in the presence of IFNβ for 16 hours. Cross-linked protein-DNA complexes were precipitated by addition of antibodies against PU.1 or ICSBP to the cell lysates and analyzed by PCR for the presence of the Ink4b promoter region or for the c-myb promoter region as a negative control. As an additional negative control, normal rabbit serum or isogenic IgGs was used for precipitation.