Ionizing radiation (IR) induces sustained stimulation of the DNA damage response pathway. ( A) Tumour tissue extracted from Control and IR–treated animals were subjected to immunoblotting analysis using ATM, P-Chk2 and γH2AX (Ser139) antibodies. Anti-actin was used as a loading control. Representative immunoblots from 6 independent experiments are shown. ( B) Immunoblot densitometric values are shown as percent change in protein expression relative to the control group (*p < 0.05).