ATM and Seckel syndrome: Two independent ATM−/− cell lines (GM01588A and GM02052F, each expressing no ATM due to an early stop codon) and one ATR-deficient cell line (Seckel syndrome, GM18366D, expressing low levels of ATR due to alternate splicing), as well as a wildtype fibroblast line (GM05757B), were procured from the Coriell Institute.