One way the ‘male hormone’ testosterone works is by binding to the androgen receptor. Researchers at the Technical University Munich (TUM) and the Max Planck Institute for Biological Intelligence have succeeded in breeding chickens without the androgen receptor for the first time. This allowed them to study how androgen signaling affects development and appearance: animals of both sexes are infertile. In addition, some – but not all – outer physical traits are underdeveloped. This illustrates the importance of testosterone for both sexes. However, it also shows that the androgen signaling pathway alone does not make a rooster.