Table 1.

Gap formation in MEF cell extracts as scored by oligonucleotide hydbridization assay

Components	Exp. 1		Exp. 2
	Mismatch repair, %	Excision-gap signal, arbitrary units	Mismatch repair, %	Excision-gap signal, arbitrary units
Mlh1−/−Exo1−/− extract	7–8	4–5	7–8	7 ± 4
Extract (0 time)	ND	3–4	ND	3–7
Extract + Exo1	4–7	4–5	ND	5 ± 3
Extract + MutLα	23–24	5–5	30–33	9 ± 2
Extract + MutLα + Exo1	58–69	20–26	54–60	23 ± 4

Reactions containing 3′ G–T heteroduplex DNA and 120 μg (Exp. 1) or 180 μg (Exp. 2) of Mlh1^−/− Exo1^−/− MEF cell extract were performed as described in Fig. 1 in the absence (to score repair) or presence of 50 μM aphidicolin (to score excision). Gaps resulting from excision were visualized by hybridization of a ³²P-oligonucleotide expected to hybridize to the exposed viral DNA strand within the region spanning the original location of the mismatch (see Materials and Methods). Hybridization results (Excision-gap signal) are expressed as specific activities based on radiolabel and DNA quantification. Results are shown as the range or mean (±1 SD) obtained from 2 or at least 3 independent measurements, respectively. For purposes of comparison, hybridization values obtained in Exp. 1 were normalized to those of Exp. 2 by using the mean value obtained in the presence of MutLα and ExoI as the base. ND, not done.