The scientists knew from earlier experiments that FOXN1 is important for the embryonic development of the thymus, and speculated that it might also rejuvenate the organ in older animals. They bred a special strain of mice whose FOXN1 production could be stimulated specifically in the thymus by tamoxifen, a drug more familiar as a treatment for breast cancer.
In one-year-olds, stimulating FOXN1 production in the thymus caused it to become 2.7 times bigger within a month. In two-year-olds the increase was 2.6 times. Moreover, when the researchers studied the enlarged thymuses microscopically, and compared them with those from untreated control animals of the same ages, they found that the organs’ internal structures had reverted to their youthful nature.