DDR plays a major role in Dox-triggered cytotoxicity in GCB-DLBCL, but not ABC-DLBCL, cell lines. (A) Immunoblot analysis of DDR markers during a 24-hour Dox treatment course in cell lines representing the GCB (Val and Karpas-422) and ABC (Ly3 and Riva) subtypes. (B) Immunoblot analysis of DDR markers in response to ionizing radiation (IR; 1.4 Gy/min) at the indicated points in 1 representative GCB (Val) and 2 representative ABC (Ly3 and Riva) cell lines. In A and B, vertical gray lines demarcate samples from different cell lines that were loaded on the same gel. >, a shorter ATM protein that is likely a degradation product of ATM.⁵⁶  CHK2 and H2AX phosphorylation (C) and apoptosis (D) after a 20-hour Dox treatment with or without a 1-hour preincubation with the ATM inhibitor (ATMi), KU-55933. Results shown in the bar graphs are mean ± SD and are representative of 2 independent experiments. Two-tailed Student t test was used for pairwise comparison as indicated. *P < .05; **P < .01; ns, not significant. nil, vehicle control. Dotted horizontal lines mark the basal level of apoptosis in untreated samples.