Yong-Chun Yu at Fudan University in Shanghai, China, and his colleagues studied mice that had learned a fearful memory and were then trained to forget it. After this 'extinction' training, fear memories often come back spontaneously with time or in response to a stimulus. But the team found that this later recurrence was reduced when embryonic neurons that make a neurotransmitter called GABA were transplanted into the animals' brains two weeks before the extinction training.