ATM is essential for cell survival in response to CNDAC. (A) AT-C (○, ATM-deficient), AT-V (Δ, AT-C transfected with vector), and AT-AT (♦, AT-C repleted with full-length ATM cDNA) cells were exposed to a range of concentrations of CNDAC for 24 hours and incubated for 10 to 11 days after washing out of drugs to allow colonies to grow. (B) p53^+/+ HCT116 cells were transfected with siRNAs described in “Depletion of ATM by small interfering RNA,” and ATM protein levels were monitored by immunoblotting for 3 days. Cells were replated for clonogenic assay 1 day after transfection. Cells were exposed to CNDAC for 24 hours and incubated for 5 to 7 days after washing. (A-B) Representative data of 3 independent experiments. Points are mean ± SD of triplicate plates. (C) OCI-AML3 cells were treated with KU55933 (1μM or 5μM) 1 hour before addition of CNDAC at a variety of concentrations. Cells were incubated for an additional 24 hours before being washed into drug-free medium. Subsequently, cells were counted and diluted into methylcellulose with KU55933 (left) or drug-free medium (right) to form colonies in 8 to 9 days. The survival curves in each panel are mean ± SD values from 1 of 3 independent experiments. *P < .05; **P < .01 versus CNDAC alone. (D) AML primary cells from archived samples were incubated with KU55933 (2.5μM) 1 hour before addition of CNDAC at the indicated concentrations. Drugs were continuously present in methylcellulose medium during formation of colonies in 5 to 6 days. The survival curves are mean values of 4 patient samples (13-16), each with triplicates.