NBN and Nijmegen breakage syndrome: Since mutations in the Mre11-Rad50-Nbn (MRN) complex give rise to a chromosomal hypersensitivity to both IR and MMC and, at least in the case of NBS patients predisposition to developing lymphoid malignancies, such as B cell lymphomas, we monitored the activation of ATM in response to DSBs using Western blotting coupled with phospho-antibodies to known substrates of ATM.