Scientists at Carnegie Mellon University in Pittsburgh have created a transgenic mouse that will allow researchers to visualize patterns of activity directly in individual neurons in vivo.[1] To create the animals, Alison Barth and colleagues coupled the c-fos promoter, which is typically activated during neural activity, to a green fluorescent protein (GFP) marker. "By coupling c-fos activation to the expression of GFP, I could now see cells that were specifically implicated by particular stimuli, and specifically stimuli in vivo," Barth explains.