ATM activity was stimulated by infection, as indicated by phosphorylated Nbs1 and Chk2 in western blots (Figure 2B, compare lane 1 to lane 5), reduced by the presence of Ku-55933 in either the early or late phase of infection (Figure 2B, compare lanes 2, 3 to lane 1), and nearly abolished by the presence of Ku-55933 throughout infection (Figure 2B, lane 4). This evidence concerns the gene CHEK2 and infection.