Table 1.

Mutation at Aprt

Cell line	Mutation frequency at Aprt	Mutation rate for Aprt	Percent of Aprt− colonies caused by mitotic recombination	Percent of Aprt− colonies caused by nondisjunction
3C5	2.2  × 10−7	8.3  × 10−8	26	53
2B4	2.8  × 10−7	8.8  × 10−8	28	52
1837	4.2  × 10−7	1.0  × 10−7	NA	NA

 The mutation frequencies and rates for Aprt were determined for each of the three ES cell clones as described in Methods for Aprt. The clones 3C5 and 2B4 were derived from a cross between mouse strains 129^SvEv and C3H/HeJ. The 1837 clone was solely of 129^SvEv origins. All Aprt colonies manifesting mitotic recombination or nondisjunction events had lost the untargeted Aprt allele. These colonies arising as a consequence of mitotic recombination had lost polymorphic markers flanking Aprt but had retained heterozygosity at proximal markers. Aprt⁻ colonies arising as a consequence of nondisjunction had lost heterozygosity at all chromosome-8 markers. Nondisjunction was verified in colonies derived from clone 3C5 by fluorescence in situ hybridization analysis and centromere size. Mitotic recombination and nondisjunction events could not be analyzed in colonies derived from clone 1837 because it is homozygous throughout except at the Aprt locus. NA, not applicable.