Myc challenges tumor-suppressive Atm signaling in the B-cell compartment in vivo. (A) Immunoblot analysis of the Atm-governed DDR signature consisting of Atm, Atm-P-S1987, p53, p53-P-S18, and caspase-3 with its proapoptotic cleavage product in lysates of immunobead-selected Atm-proficient and -deficient nontransgenic and preneoplastic Eμ-myc transgenic B cells with tubulin as a loading control. (B) Spontaneous apoptosis of Atm^+/+ and Atm^−/− nontransgenic and preneoplastic Eμ-myc transgenic B cells in cytospin preparations (n = 4) quantified by fluorescence-based TUNEL staining. Data are expressed as mean plus or minus SD; *P < .05. (C) Lymphoma incidence in Eμ-myc transgenic mice in an Atm^+/+ (control [ctrl]; n = 44; black), Atm^+/− (n = 74; blue), and Atm^−/− (n = 44; red) background. (D) Histopathological presentations of Eμ-myc control and Atm^−/− lymphomas. Representative photomicrographs of hematoxylin and eosin–stained tissue sections of the indicated organs obtained at manifestation. Original magnification, × 200.