chop-deficient cells have enhanced survival when challenged with toxins that induce ER stress. (A) Western blots of CHOP and TLS (an internal control) proteins in lysates from MEFs with the indicated chop genotype following 6 hr of treatment with tunicamycin (TUN, 1 μg/ml), thapsigargin (TG, 2 μm), and calcium ionophore (A23187, 2 μm). (B) Phase-contrast photomicrograph of MEFs treated for 24 hr with the indicated compounds (320×). (C) Quantification of the fraction of dead cells as a function of time in tunicamycin treated MEFs with the indicated genotypes. (D) Survival of cells plated at low density and treated for 24 hr with the indicated dose of tunicamycin and then changed to normal growth media. At each dose, the number of colonies at 10 days in the treated plates is compared with the number of colonies in an untreated plate of the same density. Shown are mean and s.d. of a typical experiment performed in triplicate and repeated three times using independently prepared pools of MEFs.