Double transgenic mice were cross bred with wild type mice, and homozygous Tbr2^f/f and Smad4^f/f mice. The excision event was determined by the expression of respective reporter gene activities or by PCR of the excised floxed alleles as described in Methods. Cre^Tg: hemizygous Cre transgenic mice; Cre⁰: non-Cre transgenic mice. The asterisk indicates that the ZEG transgene is on chromosome 11, the same as Krt12. Thus, the ZEG allele is always co-segregated with the Krt12^Cre allele. The sharp (hash mark) indicates homozygous floxed Tbr2 and Smad4 alleles and the double sharp indicates that the percent of penetrance is calculated by number of mice with excised floxed allele divided by the total number of mice carrying floxed allele in each experimental group.