FIG. 4.

Frequencies of SCEs in lymphocytes deficient for PARP1 and/or 53BP1. (A) B cells of the indicated genotypes were activated with α-CD40 plus IL-4 for 24 h and incubated in 10 μM BrdU for an additional 24 h (two cell divisions), and metaphase spreads were hybridized with a FITC-labeled anti-BrdU antibody. For studies of drug-induced SCEs, metaphases were obtained after exposure to 10 ng/ml MMC for 24 h or 0.25 mM HU for 4 h, followed by a washout period of 8 h. For all experiments, colcemid was used at a concentration of 0.05 μg/ml for 2 h prior to cell harvesting. The histograms represent the number of SCEs per chromosome; n is the number of chromosomes analyzed for each condition. (B) Examples of HU-treated metaphases of the indicated genotypes, with representative chromosomes magnified in the insets.