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.
