Removing a single gene from the brains of mice and zebrafish causes these animals to become more anxious than normal. Researchers show that eliminating the gene encoding Lef1 disrupts the development of certain nerve cells in the hypothalamus that affect stress and anxiety. These results are the first implication that Lef1 functions in this brain region to mediate behavior, knowledge that could one day prove useful for diagnosing and treating human brain disorders.