In situ DIG-dUTP incorporation in mouse cells transfected with the GFP or GFP-Tax expressing plasmids. Visualisation of the GFP (green fluorescence, a) or GFP-Tax (green fluorescence, b, b1, b2) signals. Counter-staining with DAPI (blue fluorescence). Visualisation of DIG-dUTP incorporation (red fluorescence) in the absence (a1) or in the presence (b, b1, b2) of Tax. In situ incorporation was obtained using DIG-dUTP which is incorporated at the free 3-′OH DNA ends and visualised with rhodamine conjugated anti-DIG antibodies. GFP-Tax visualisation traces the presence of Tax (green fluorescence). The Tax signal and the DIG-dUTP incorporation signal in the same nucleus may coincide (b, b2) or may be separate (b1). MN are visible in b2 (white arrows); one MN carries the incorporation signal (red). a, a1, b show H2AX−/− cells; b1, b2 show XRCC4−/− cells. The size of incorporation signal is larger in the presence of Tax (b, b1) than in the absence (a1).