Transient silencing of Id1 in transformation assays. Rat-1 cells were transfected with negative control (panels A, B, E, F) or Id1 siRNA (panels C, D, G, and H) in twelve-well plates. The following day cells were transduced with control HSCG retrovirus (panels A, C, E, G), or LMP1-expressing retrovirus, HSCG-LMP1 (panels B, D, F, H) and focus formation assays were performed for 7 days. Monolayers were fixed, stained with crystal violet, and examined for focus formation by light microscopy using a dissecting microscope. Representative micrographs are depicted at 10X (panels A, B, C, D) or 40X (panels E, F, G, H).