In a female mouse born with two wild-type Foxp3 alleles, what is the minimum number of inactivating, recessive mutations that might be sufficient in the Foxp3 alleles in mammary epithelium to significantly increase the likelihood that this mouse will develop mammary tumors?

The correct answer is: one mutation.

Since the FoxP3 gene is located on the X chromosome, one recessive mutation in female Foxp3 allele (two X chromosomes) will be sufficient in the mammary epithelium to significantly increase the likelihood for development of mammary tumors. 60% of mice which carry 1 deactivating scruffin (sf) and one WT (+) allele will spontaneously develop tumor.