DNA strand breaks were demonstrated by labeling free 3′-OH termini with FITC-labeled deoxyuridine, which was detected with alkaline phosphatase–coupled, anti-fluorescein antibody, and the formation of a dye precipitate with a phosphatase substrate (In Situ Cell Death Detection Kit, AP; Boehringer Mannheim, Mannheim, Germany). Yeast cells were fixed with 3.7% formaldehyde, digested with lyticase, and applied to a polylysine-coated slide as described for immunofluorescence (Adams and Pringle, 1984 (link)). The slides were rinsed with PBS, incubated in permeabilization solution (0.1% Triton X-100, 0.1% sodium citrate) for 2 min on ice, rinsed twice with PBS, incubated with 10 μl TUNEL reaction mixture (200 U/ml terminal deoxynucleotidyl transferase, 10 mM FITC-labeled dUTP, 25 mM Tris/HCl, 200 mM sodium cacodylate, 5 mM cobalt chloride; Boehringer Mannheim) for 60 min at 37°C, rinsed three times with PBS, incubated with 50 μl Converter AP solution (alkaline phosphatase– labeled, anti-FITC antibody; Boehringer Mannheim) for 30 min at 37°C, rinsed three times with PBS, and stained by incubation with 50 μl naphthol) AS-MX phosphate (Sigma Chemical Co., Munich, Germany), 0.8 mg/ml, fast red TR salt (Sigma Chemical Co.), 1 mg/ml, 2% dimethylformamide, 1 mM levamisole in 100 mM Tris/HCl, pH 8.2, for 30 min at room temperature. A coverslip was mounted with a drop of Kaiser's glycerol gelatin (Merck, Darmstadt, Germany).