Representative images depicting the level of DNA double strand break (DSB) induction in histologically normal bone marrow 5h after treatment, as measured by anti-ɣ-H2AX, which is higher in free oligo-FdU-treated mice than in T22-GFP-H6-FdU (P=0.047). Low level of cells containing DSBs in histologically normal kidney after T22-GFP-H6-FdU or free oligo-FdU treatment, a finding occurring in all normal tissues analysed. (N=50, 5 mice/group; 10 fields/mouse). 