Logarithmically growing apn1/2 and triple mutants apn1/2 mre11 or apn1/2 rad50 were arrested in G2/M with nocodazole, treated with MMS (0.1%, 15 min) in PBS and returned to the YPDA+nocodazole medium. Cells were collected at the indicated times and processed for PFGE analysis. Chromosomes were visualized by ethidium bromide staining (A) and by Southern blotting (B) with a CHA1 probe that identifies Chr III. Resection-related PFGE-shift of linear Chr III and formation of Chr III dimers are indicated (B). The SMD was estimated in the ethidium bromide stained gel (A) by comparing the amount of DNA material in the SMD region to DNA in the small chromosomes that experienced little breakage, “SMD / Chr (I+VI).”