ATM and cancer: Further SMG1 and ATM are both required for maximal activation of the G1/S checkpoint following exposure to ionising radiation or during oxidative stress.13, 14 SMG1 can also regulate alternative splicing of p53 in response to DNA damage.15 To further examine the interplay between SMG1 and ATM in cancer development, we crossed mice heterozygous for both the Smg1 genetrap allele (Smg1gt/+) and the Atm null allele to generate Atm knockout mice that were also heterozygous for Smg1 (Atm−/−Smg1gt/+).